POWER 权力

Construction of ${Ti}_{3} C_{2}$ MXene@C@SnS with layered rock stratum structure for high-performance lithium storage
构建具有层状岩层结构的 ${Ti}_{3} C_{2}$ MXene@C@SnS，实现高性能锂存储

Hong Tang $^{a}$ , Ronghui Guo $^{a,^{*}}$ , Mengjin Jiang $^{b}$ , Yue Zhang $^{a}$ , Xiaoxu Lai $^{a}$ , Ce Cui $^{a}$ ,
Hong Tang $^{a}$ , Ronghui Guo $^{a,^{*}}$ , Mengjin Jiang $^{b}$ , Yue Zhang $^{a}$ , Xiaoxu Lai $^{a}$ , Ce Cui $^{a}$ 、Hongyan Xiao $^{a}$ , Shouxiang Jiang $^{c}$ , Erhui Ren $^{a}$ , Qin Qin $^{a}$
肖红艳 $^{a}$ , 蒋守祥 $^{c}$ , 任尔辉 $^{a}$ , 秦琴 $^{a}$ $^{a}$ College of Biomass Science and Engineering, Sichuan University, Chengdu, 610065, China
$^{a}$ 四川大学生物质科学与工程学院，中国成都，610065 $^{b}$ College of Polymer Science & Engineering and State Kjieey Lab Polymer Material & Engineering, Sichuan University, Chengdu, 610065, China
$^{b}$ 四川大学高分子科学与工程学院、高分子材料与工程国家重点实验室，成都，610065 $^{c}$ Institute of Textiles and Clothing, The Hong Kong Polytechnic University, China
$^{c}$ 中国香港理工大学纺织及制衣研究所

H I G H L I G H T S

${Ti}_{3} C_{2}$ MXene@C@SnS hybrids with layered rock stratum structure was successfully prepared.
<成功制备了具有层状岩层结构的 MXene@C@SnS 混合物。
${Ti}_{3} C_{2}$ MXene@C@SnS hybrids have large surface area ( $255.78 m^{2} g^{- 1}$ ) and porosity.
${Ti}_{3} C_{2}$ MXene@C@SnS杂化物具有较大的表面积（ $255.78 m^{2} g^{- 1}$ ）和孔隙率。
The electrode has high reversible capacity of $1473 mA h g^{- 1}$ at $0.1 A g^{- 1}$ .
该电极在 $0.1 A g^{- 1}$ 时具有 $1473 mA h g^{- 1}$ 的高可逆容量。
The electrode has excellent rate performance of $640 mA h g^{- 1}$ at $5 A g^{- 1}$ .
该电极在 $5 A g^{- 1}$ 时具有 $640 mA h g^{- 1}$ 的优异速率性能。
Outstanding cycle stability of 1050 mA $h g^{- 1}$ over 350 cycles at $1 A g^{- 1}$ .
在 $1 A g^{- 1}$ 的 350 个周期内， $h g^{- 1}$ 达到 1050 mA 的出色周期稳定性。

A R T I C L E I N F O

Keywords: 关键词：

Lithium-ion storage 锂离子存储

{Ti}_{3} C_{2}

MXene@C@SnS hybrids

{Ti}_{3} C_{2}

MXene@C@SnS杂化物
Layered rock stratum structure
层状岩层结构
Rate performance 费率性能
Long-cycle stability 长周期稳定性
GR A P H I C A L A B S T R A C T
训研所

A B S T R A C T

Tin sulfide ( SnS ) possesses high theoretical capacity and make it a very potential anode material for lithium-ion batteries. Nevertheless, the poor electrical conductivity of SnS is prone to collapse during lithiation/de-lithiation. Herein,

{Ti}_{3} C_{2}

MXene@C@SnS hybrids with layered rock stratum structure are prepared as an anode electrode for lithium ion batteries through hydrothermal and subsequent annealing. The hybrids integrate large specific surface area and porosity, and accelerate electron/ion transfer. The

{Ti}_{3} C_{2}

MXene@C@SnS anode exhibits a superior capacity (

1473 mA g^{- 1}

0.1 {Ag}^{- 1}

), outstanding rate capability (

640 mA g^{- 1}

5 A g^{- 1}

and keeps

1142.2 mA g g^{- 1}

for 70 cycles returning to

0.5 A g^{- 1}

again) and excellent long-cycle stability (

1050 mA h g^{- 1}

1 A g^{- 1}

over 350 cycles). Kinetic analysis reveals that the excellent rate capability is controlled by surface pseudocapacitance behavior at high current. This result indicates that

{Ti}_{3} C_{2}

MXene@C@SnS can be potentially applied in the field of lithium storage.
硫化锡（SnS）具有很高的理论容量，是一种非常有潜力的锂离子电池负极材料。然而，SnS 的导电性较差，在锂化/去锂化时容易发生塌陷。在此，我们通过水热法和随后的退火法制备了具有层状岩层结构的

{Ti}_{3} C_{2}

MXene@C@SnS杂化物，作为锂离子电池的负极电极。该杂化物具有较大的比表面积和孔隙率，可加速电子/离子传输。

{Ti}_{3} C_{2}

MXene@C@SnS阳极表现出卓越的容量（

0.1 {Ag}^{- 1}

时为

1473 mA g^{- 1}

）、出色的速率能力（

5 A g^{- 1}

时为

640 mA g^{- 1}

并保持

1142.2 mA g g^{- 1}

70次循环后再次回到

0.5 A g^{- 1}

）和出色的长循环稳定性（

1 A g^{- 1}

时为

1050 mA h g^{- 1}

超过350次循环）。动力学分析表明，卓越的速率能力是由大电流时的表面假电容行为控制的。这一结果表明，

{Ti}_{3} C_{2}

MXene@C@SnS 有可能应用于锂存储领域。

1. Introduction 1.导言

Lithium-ion batteries (LIBs) are a very important type of rechargeable batteries due to their high energy density, long lifetime, no memory effect, good safety and environmental benignity, so it has been widely used in various portable electronic devices, new energy electric vehicles and power-grid systems

[1 - 4]

. However, LIBs are meeting with urgently need on energy density increase, long cycle longevity and low cost when
锂离子电池（LIB）是一种非常重要的可充电电池，由于其能量密度高、寿命长、无记忆效应、安全性好且对环境无害，已被广泛应用于各种便携式电子设备、新能源电动汽车和电网系统

[1 - 4]

。然而，锂离子电池在提高能量密度、长周期寿命和低成本方面的需求十分迫切。

https://doi.org/10.1016/j.jpowsour.2020.228152
Received 9 January 2020; Received in revised form 28 March 2020; Accepted 2 April 2020
Available online 20 April 2020
0378-7753/© 2020 Elsevier B.V. All rights reserved.
they are further applied in the field of electric vehicles and power-grid systems [5-7]. The electrochemical performance of LIBs usually depends on anode materials, but the commercial graphite anode only has a

from meeting the ever-growing demand of energy. Therefore, exploring lithium storage anode materials with high capacity becomes increasingly urgency.
能源需求不断增长。因此，探索高容量锂储能正极材料变得日益迫切。

Among various anode materials, SnS as transition metal sulfide, are very promising as a replacement for lithium-ion battery anodes because of its high theoretical capacity (

1368 mA h^{- 1}

) provided by the unique alloying conversion during charge and discharge. Nevertheless, SnS suffers from inferior capacity and rate performance because of intrinsic poor electrical conductivity and structural instability. In addition, the SnS lattice collapse easily during large-scale lithiation/de-lithiation, resulting in fast capacity decay and inferior rate performance [10].
在各种负极材料中，作为过渡金属硫化物的 SnS 很有希望成为锂离子电池负极的替代材料，因为它在充放电过程中通过独特的合金转换提供了很高的理论容量（

1368 mA h^{- 1}

）。然而，SnS 因其固有的低导电性和结构不稳定性，在容量和速率性能方面都较差。此外，在大规模锂化/去锂化时，SnS 晶格容易坍塌，导致容量快速衰减和速率性能降低 [10]。

To overcome the above obstacles, construction of nanostructured SnS composites has been proven to significantly improve electrochemical performance. Especially, SnS nanoparticles can provide a large number of active sites due to their high specific surface area. The SnS nanoparticles can effectively shorten the lithium ion diffusion distance and increase the specific surface area to enlarge the active sites of lithium ion insertion and extraction [11,12]. In addition, anchoring SnS nanoparticles on conductive matrices was an effective strategies to improve intrinsic electrical conductivity and prevent SnS nanoparticles from aggregating. Currently, SnS nanoparticles have been anchored on various conductive matrices such as graphene [10] and carbon nanosheet [13], which greatly improves capacity of SnS nanoparticle and cycle stability. However, graphene and carbon nanosheets exhibits slow lithium ion diffusion and depressed reaction dynamics when being used as anode materials for lithium-ion batteries [14]. Compared with graphene and carbon nanosheets, the two-dimensional (2D)

{Ti}_{3} C_{2}

MXene has high diffusion mobility and low diffusion energy barrier of lithium ions [15]. The

{Ti}_{3} C_{2}

MXene has fast electron transfer rate toward accelerating reaction dynamics. Therefore,

{Ti}_{3} C_{2}

MXene can be used as an alternative conductive matrice for growth of SnS nanoparticles for lithium ion storage. Recently,

SnS / PDDA - {Ti}_{3} C_{2}

composite was prepared by electrostatic attraction, which showed lithium ion storage capacity of

795 mA h g^{- 1}

50 mA g^{- 1}

. However, SnS/PDDA-Ti

_{3} C_{2}

composite did not show the superiority of SnS with high capacity (

1368 mA h g^{- 1}

) [16]. Additionally,

{Ti}_{3} C_{2}

MXene tends to be automatically converted to stable metal oxides, which results in reduction of the electrical conductivity and diffusion mobility of lithium ions. Previous studies have proven that carbon encapsulation could efficiently alleviate the oxidation of MXene [17]. However, carbon-encapsulated SnS nanoparticles anchored on multilayer

{Ti}_{3} C_{2}

MXene by in-situ growth as anode electrode material for LIBs with high-performance lithium storage have not been reported.
为克服上述障碍，纳米结构 SnS 复合材料的构建已被证明可显著提高电化学性能。尤其是 SnS 纳米粒子的高比表面积可以提供大量的活性位点。SnS 纳米颗粒能有效缩短锂离子扩散距离，增加比表面积，从而扩大锂离子插入和萃取的活性位点 [11,12]。此外，将 SnS 纳米粒子锚定在导电基质上也是提高其内在导电性和防止 SnS 纳米粒子聚集的有效策略。目前，已有人将 SnS 纳米粒子锚定在石墨烯[10]和碳纳米片[13]等多种导电基质上，从而大大提高了 SnS 纳米粒子的容量和循环稳定性。然而，石墨烯和碳纳米片在用作锂离子电池的负极材料时，锂离子扩散速度较慢，反应动力学受抑制[14]。与石墨烯和碳纳米片相比，二维（2D）

{Ti}_{3} C_{2}

MXene 具有较高的扩散迁移率和较低的锂离子扩散能垒[15]。

{Ti}_{3} C_{2}

MXene 具有快速的电子转移率，可加速反应动力学。因此，

{Ti}_{3} C_{2}

MXene 可用作生长 SnS 纳米粒子的替代导电材料，用于锂离子存储。最近，利用静电吸引法制备了

SnS / PDDA - {Ti}_{3} C_{2}

复合材料，在

50 mA g^{- 1}

条件下显示出

795 mA h g^{- 1}

的锂离子存储容量。然而，SnS/PDDA-Ti

_{3} C_{2}

复合材料并没有显示出 SnS 高容量（

1368 mA h g^{- 1}

）的优越性[16]。此外，

{Ti}_{3} C_{2}

MXene 往往会自动转化为稳定的金属氧化物，从而导致锂离子的导电性和扩散迁移率降低。以往的研究证明，碳包覆可以有效缓解 MXene 的氧化作用 [17]。然而，通过原位生长将碳包封的 SnS 纳米粒子锚定在多层

{Ti}_{3} C_{2}

MXene 上，作为具有高性能储锂功能的 LIB 的正极电极材料的研究还未见报道。

In this work, the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids with layered rock stratum structure were synthesized by a novel annealing reduction strategy. In the presence of glucose, carbon encapsulation nanoparticleslike

{SnS}_{2}

was grown in situ on the surface and interlayer of multilayer

{Ti}_{3} C_{2}

MXene by hydrothermal treatment, and

{SnS}_{2}

was then reduced to SnS after annealing. The illustration of synthesis of MXene@C@SnS hybrids is shown in Fig. 1. The

{Ti}_{3} C_{2}

MXene@C@SnS hybrids possess large specific surface area to provide plentiful active sites for the electrochemical reaction and rich porous structure can provide numerous ion transport channels to significantly shorten the diffusion channel of electrolyte. The carbon nanolayers effectively improve electrical conductivity and structural stability. As a result,

{Ti}_{3} C_{2}

MXene@C@SnS electrode exhibits excellent lithium storage performance when being employed as lithium-ion battery anode. The electrode delivers a remarkable reversible capacity of

1473 mA h^{- 1}

0.1 {Ag}^{- 1}

, especially outstanding rate capability of

640 mA h^{- 1}

at a high current density of

5 A g^{- 1}

and maintaining a capacity of

1142.2 mA h^{- 1}

for 70 cycles returning to

0.5 A g^{- 1}

again. The electrode still delivers a reversible capacity up to

1050 mA h g^{- 1}

with

99 %

Coulombic efficiency for 350 cycles at a high current density of

1 A g^{- 1}

.
本研究采用新型退火还原策略合成了具有层状岩层结构的

{Ti}_{3} C_{2}

MXene@C@SnS杂化物。在葡萄糖存在的条件下，通过水热处理在多层

{Ti}_{3} C_{2}

MXene的表面和层间原位生长出类似

{SnS}_{2}

的碳封装纳米颗粒，然后将

{SnS}_{2}

退火还原成SnS。MXene@C@SnS 混合物的合成示意图如图 1 所示。

{Ti}_{3} C_{2}

MXene@C@SnS杂化物具有较大的比表面积，为电化学反应提供了丰富的活性位点，丰富的多孔结构提供了众多的离子传输通道，大大缩短了电解质的扩散通道。碳纳米层可有效提高导电性和结构稳定性。因此，

{Ti}_{3} C_{2}

MXene@C@SnS 电极在用作锂离子电池负极时具有优异的储锂性能。该电极在

0.1 {Ag}^{- 1}

条件下具有

1473 mA h^{- 1}

的显著可逆容量，尤其是在

5 A g^{- 1}

的高电流密度条件下具有

640 mA h^{- 1}

的出色速率能力，并能在 70 个循环中保持

1142.2 mA h^{- 1}

的容量，再次回到

0.5 A g^{- 1}

。在

1 A g^{- 1}

的高电流密度下，该电极在 350 个循环中仍能提供高达

1050 mA h g^{- 1}

的可逆容量和

99 %

的库仑效率。

2. Experimental section 2.实验部分

2.1. Preparation of multilayer ${Ti}_{3} C_{2}$ MXene
2.1.制备多层 ${Ti}_{3} C_{2}$ MXene

The

{Ti}_{3} C_{2}

MXene was prepared by selectively etching 1.0 g of

{Ti}_{3} {AlC}_{2}

MAX powder (Forsman Scientific (Beijing) Co., Ltd.) in

40 % HF

solutions ( 20 mL ) to remove Al layers at

60^{\circ} C

for 48 h . The

{Ti}_{3} C_{2}

MXene dispersion solution was obtained by several centrifugal separations with distilled water until the pH was nearly 7 to entirely remove the residual HF. Finally, the multilayer

{Ti}_{3} C_{2}

MXene powder was collected after freeze-drying.
在

40 % HF

溶液（20 mL）中选择性地蚀刻 1.0 g

{Ti}_{3} {AlC}_{2}

MAX 粉末（福斯曼科技（北京）有限公司），以去除

60^{\circ} C

处的铝层，48 h 后制备出

{Ti}_{3} C_{2}

MXene。

{Ti}_{3} C_{2}

MXene 分散液用蒸馏水离心分离几次，直到 pH 值接近 7，以完全去除残留的 HF。最后，在冷冻干燥后收集多层

{Ti}_{3} C_{2}

MXene 粉末。

2.2. Synthesis of $T i_{3} C_{2}$ Mxene@C@SnS hybrids
2.2.合成 $T i_{3} C_{2}$ Mxene@C@SnS 杂化物

40 mg Ti_{3} C_{2}

MXene powder was ultrasonically dispersed in 40 mL distilled water to obtain homogeneous

{Ti}_{3} C_{2}

MXene dispersion solution (

1 mg / mL, 40 mL)

. Afterwards,

{SnCl}_{4} \cdot 4 H_{2} O (224 mg)

, thioacetamide (TAA, 168 mg ) and glucose ( 400 mg ) were dissolved in

{Ti}_{3} C_{2}

MXene dispersion solution. The obtained mixture was dispersed under ultrasonic irradiation for 30 min . After that, the mixture solution was moved
将

40 mg Ti_{3} C_{2}

MXene 粉末超声分散在 40 mL 蒸馏水中，得到均匀的

{Ti}_{3} C_{2}

MXene 分散液（

1 mg / mL, 40 mL)

。然后，在

{Ti}_{3} C_{2}

MXene 分散液中溶解

{SnCl}_{4} \cdot 4 H_{2} O (224 mg)

、硫代乙酰胺（TAA，168 毫克）和葡萄糖（400 毫克）。得到的混合物在超声波照射下分散 30 分钟。然后，将混合物溶液

Fig. 1. Illustration of synthesis of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids.
图 1.

{Ti}_{3} C_{2}

MXene@C@SnS 混合物的合成示意图。
into a 50 mL stainless-steel Teflon-lined autoclave and maintained in a furnace at

180^{\circ} C

for 24 h . The

{Ti}_{3} C_{2}

MXene@C@

{SnS}_{2}

hybrids were obtained after washing with distilled water for several times, and then vacuum dried in an oven at

60^{\circ} C

for 12 h . Subsequently, the obtained

{Ti}_{3} C_{2}

MXene@C@SnS

_{2}

hybrids was annealed in Ar flow at a heating rate of

2^{\circ} C \min^{- 1}

500^{\circ} C

for 2 h and the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids were formed. For comparison, the C@SnS hybrids were synthesized under similar conditions without

{Ti}_{3} C_{2}

MXene. All active substances were not purified before the electrochemical test.
在 50 mL 不锈钢特氟龙内衬高压釜中，在

180^{\circ} C

炉中保持 24 小时。

{Ti}_{3} C_{2}

MXene@C@

{SnS}_{2}

混合物经蒸馏水多次洗涤后得到，然后在

60^{\circ} C

烘箱中真空干燥 12 小时。随后，将得到的

{Ti}_{3} C_{2}

MXene@C@SnS

_{2}

杂化物在氩气流中以

2^{\circ} C \min^{- 1}

的加热速率在

500^{\circ} C

下退火 2 小时，形成

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物。为了进行比较，在类似条件下合成了不含

{Ti}_{3} C_{2}

MXene 的 C@SnS 杂化物。所有活性物质在电化学测试前均未提纯。

2.3. Materials characterization
2.3.材料特征

Morphologies of the sample was observed by SEM (JEOL, JSM5900LV) and TEM (JEOL JEM2010). The elemental mapping images were tested by TEM (JEOL JEM2010). The SEAD were carried out by ACTEM (FEI Titan G2 60-300). XRD spectra is conducted by a D/Max-III Xray spectrometer (PANalytical/Philips X’Pert Pro) when

2 θ

values were ranged from

5^{\circ}

80^{\circ}

with

Cu K α

radiation. Raman spectroscopy was studied by Raman (Andor SR-500i) with 532 nm laser excitation. XPS were investigated using electron energy disperse spectroscopy (Thermo fisher Scientific Escalab 250Xi, USA). Nitrogen sorption-desorption isotherms were measured by using an adsorption apparatus (Gemini VII 2390).
样品的形态由 SEM（JEOL，JSM5900LV）和 TEM（JEOL JEM2010）观察。用 TEM（JEOL JEM2010）测试了元素图谱图像。SEAD 由 ACTEM（FEI Titan G2 60-300）进行。XRD 光谱由 D/Max-III X 射线光谱仪（PANalytical/Philips X'Pert Pro）测定，

2 θ

值范围为

5^{\circ}

至

80^{\circ}

，辐射为

Cu K α

。拉曼光谱由拉曼仪（Andor SR-500i）在 532 nm 激光激发下进行研究。XPS 采用电子能谱仪（Thermo fisher Scientific Escalab 250Xi，美国）进行研究。使用吸附仪（Gemini VII 2390）测量了氮的吸附-解吸等温线。

2.4. Electrochemical measurement
2.4.电化学测量

The as-synthesized C@SnS and

{Ti}_{3} C_{2}

Mxene@C@SnS hybrids were assembled into standard CR2032-type coin cells for the electrochemical performance test. All the electrodes were coated with slurry. Firstly, the preparation of homogeneous ink-like slurry via mixing with a mass ratio of

70 %

active materials (C@SnS and

{Ti}_{3} C_{2}

Mxene@C@SnS), 20% acetylene black (super P) and 10% sodium carboxymethyl cellulose in distilled water. The purpose of

20 %

acetylene black is to improve the electrical conductivity of external circuits [18]. Afterwards, the slurry was coated on copper foil and then quickly transferred to a vacuum oven at

60^{\circ} C

for overnight. The electrodes film was then obtained after being punched into small wafer with diameter of 12 mm . CR2032-type button cell was assembled in an argon-filled vacuum glove box with oxygen and moisture content below 0.1 ppm . The Li foil was used as a contrast electrode and Celgard 2400 (polypropylene film) as the battery separator. The electrolyte was prepared by dissolving

1.0 M {LiPF}_{6}

in mixture of dimethyl carbonate and ethylene carbonate (

1 : 1 vol / vol

). The galvanostatic charge and discharge tests were employed on a LAND CT2001A battery test system at different current densities at room temperature. Cyclic voltammetry (CV) tests were conducted using a CHI660E electrochemical workstation between 0.01 and 3.0 V with a sweep rate of

0.1 mV s^{- 1}

. EIS measurements were employed by CHI660E workstation at the frequency ranging from 100000 Hz to 0.01 Hz with
将合成的 C@SnS 和

{Ti}_{3} C_{2}

Mxene@C@SnS 混合物组装到标准 CR2032 型纽扣电池中进行电化学性能测试。所有电极都涂有浆料。首先，在蒸馏水中混合一定质量比的

70 %

活性材料（C@SnS 和

{Ti}_{3} C_{2}

Mxene@C@SnS）、20% 乙炔黑（超 P）和 10% 羧甲基纤维素钠，制备均匀的油墨状浆料。

20 %

乙炔黑的作用是提高外部电路的导电性[18]。随后，将浆料涂在铜箔上，然后迅速转移到

60^{\circ} C

真空烘箱中过夜。然后将电极薄膜冲压成直径为 12 毫米的小圆片。CR2032 型纽扣电池是在氧气和水分含量低于 0.1 ppm 的氩气真空手套箱中组装的。锂箔用作对比电极，Celgard 2400（聚丙烯薄膜）用作电池隔膜。电解液是将

1.0 M {LiPF}_{6}

溶于碳酸二甲酯和碳酸乙烯酯的混合物（

1 : 1 vol / vol

）中制备的。在 LAND CT2001A 电池测试系统上进行了电流静态充放电测试，测试条件为室温下不同的电流密度。循环伏安 (CV) 测试使用 CHI660E 电化学工作站在 0.01 至 3.0 V 之间进行，扫描速率为

0.1 mV s^{- 1}

。使用 CHI660E 工作站在 100000 Hz 到 0.01 Hz 的频率范围内进行了 EIS 测量，扫描速率为

0.1 mV s^{- 1}

。
AC amplitude of 5 mV . All the cells were placed for 24 h before testing in order to make the electrolyte better wet the electrode materials.
交流振幅为 5 mV。测试前，所有电池都放置了 24 小时，以使电解液更好地润湿电极材料。

3. Results and discussion
3.结果和讨论

X-ray diffraction (XRD) pattern (Fig. 2a and Fig. S1) was employed to analyze the crystal structure of

{Ti}_{3} {AlC}_{2}, {Ti}_{3} C_{2}, C @ SnS

and

{Ti}_{3} C_{2}

MXene@C@SnS hybrids. The Al peak at

{38.7}^{\circ}

disappears and a new peak at

{8.79}^{\circ}

is observed, which demonstrates successful preparation of

{Ti}_{3} C_{2}

MXene after

{Ti}_{3} {AlC}_{2}

is etched by HF (Fig. S1). The diffraction peaks of

{Ti}_{3} C_{2}

MXene@C@SnS

_{2}

hybrids can be well assigned to standard card of

2 T SnS_{2}

(JCPDS No. 23-0677), which proves that

{SnS}_{2}

is generated after hydrothermal treatment. All the diffraction peaks of C@SnS and

{Ti}_{3} C_{2}

MXene@C@SnS hybrids can be well indexed to the standard card of SnS (PDF card no. 33-1375) and no impurity peak appears as shown in Fig. 2a, which imply that

{SnS}_{2}

is successfully converted into SnS after annealing. Compared with the angle of the diffraction peak of

{Ti}_{3} C_{2}

MXene (002) plane at

{8.7}^{\circ}

, the

{Ti}_{3} C_{2}

MXene@C@SnS (002) plane shifts to lower angle (the diffraction peaks at

{6.8}^{\circ}

), which is resulted from the increase in the interlayer spacing by in situ formation of SnS nanoparticles between the layers and surface of the

{Ti}_{3} C_{2}

MXene. However, no obvious amorphous carbon peaks are observed in the XRD pattern of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids due to the formation of highly graphitized carbon after annealing, and the graphite carbon peak is overlapped with the diffraction peak of SnS at

26^{\circ}

[19,20]. For confirming the presence of carbon, Raman spectroscopy of

{Ti}_{3} C_{2}

MXene@C@SnS was conducted. Several characteristic peaks of

152 {cm}^{- 1}, 311 {cm}^{- 1}, 1381

{cm}^{- 1}

and

1571 {cm}^{- 1}

can be observed from the Raman spectra of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids (Fig. 2b), which correspond to B

_{3 g}

mode of SnS [21],

A_{1 g}

mode of

{SnS}_{2}

[22], disordered (D) band and graphitic (G) band, respectively. The intensity ratio of

D

band to

G

band is 0.93 . The relatively low ratio value of intensity is resulted from the formation of abundant graphitized carbon materials in

{Ti}_{3} C_{2}

MXene@C@SnS hybrids after annealing [10,23,24]. The graphitized carbon nanolayers facilitate to improve the stability of the structure and the electrical conductivity of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids.
利用 X 射线衍射（XRD）图（图 2a 和图 S1）分析了

{Ti}_{3} {AlC}_{2}, {Ti}_{3} C_{2}, C @ SnS

和

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的晶体结构。在

{38.7}^{\circ}

处的 Al 峰消失了，在

{8.79}^{\circ}

处出现了一个新的峰，这表明在

{Ti}_{3} {AlC}_{2}

被 HF 蚀刻后，成功制备出了

{Ti}_{3} C_{2}

MXene（图 S1）。

{Ti}_{3} C_{2}

MXene@C@SnS

_{2}

杂化物的衍射峰可以很好地归属于

2 T SnS_{2}

的标准卡（JCPDS 编号：23-0677），这证明

{SnS}_{2}

是在水热处理后生成的。如图 2a 所示，C@SnS 和

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的所有衍射峰都能很好地与 SnS 的标准卡（PDF 卡编号：33-1375）进行索引，并且没有杂质峰出现，这意味着

{SnS}_{2}

在退火后成功转化为 SnS。与

{Ti}_{3} C_{2}

MXene (002) 面的衍射峰

{8.7}^{\circ}

的角度相比，

{Ti}_{3} C_{2}

MXene@C@SnS (002) 面的衍射峰向更低的角度移动（衍射峰在

{6.8}^{\circ}

处），这是由于在

{Ti}_{3} C_{2}

MXene 的层间和表面原位形成了 SnS 纳米颗粒，从而增加了层间间距。然而，由于退火后形成了高度石墨化的碳，

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的 XRD 图谱中没有观察到明显的无定形碳峰，而且石墨碳峰与

26^{\circ}

处的 SnS 衍射峰重叠[19,20]。为了证实碳的存在，对

{Ti}_{3} C_{2}

MXene@C@SnS 进行了拉曼光谱分析。从

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的拉曼光谱中可以观察到

152 {cm}^{- 1}, 311 {cm}^{- 1}, 1381

{cm}^{- 1}

和

1571 {cm}^{- 1}

的几个特征峰（图 3）。 2b），分别对应于 SnS 的 B

_{3 g}

模式 [21]、

{SnS}_{2}

的

A_{1 g}

模式 [22]、无序（D）带和石墨（G）带。

D

带与

G

带的强度比为 0.93 。强度比值相对较低的原因是

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物在退火后形成了丰富的石墨化碳材料 [10,23,24]。石墨化碳纳米层有助于提高

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的结构稳定性和导电性。

The X-ray photoelectron spectroscopy (XPS) was performed to investigate the composition of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids and the result is illustrated in Fig. 3 and Fig. S2. The existence of Ti, C, S and Sn in

{Ti}_{3} C_{2}

MXene@C@SnS hybrids can be observed by survey scan of XPS as shown in Fig. S2. In addition, the presence of the O element can be observed, which is attributed to the oxygen-containing groups because the oxygen-containing groups are not completely removed from the surface of

{Ti}_{3} C_{2}

MXene after annealing. High resolution spectra of Ti 2 p ,

Sn 3 d, S 2

p and C 1s are presented in Fig. 3, respectively. In Sn 3d spectrum, two strong characteristic peaks at 495 eV and 486.4 eV are attributed to

{Sn}^{2 +} 3 d_{5 / 2}

and

{Sn}^{2 +} 3 d_{3 / 2}

, respectively. The results suggest that a large number of SnS nanoparticles exist in the hybrids, which is
为研究

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的成分，我们采用了 X 射线光电子能谱（XPS），结果如图 3 和图 S2 所示。如图 S2 所示，通过 XPS 的调查扫描可以观察到

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物中 Ti、C、S 和 Sn 的存在。此外，还可以观察到 O 元素的存在，这是由于

{Ti}_{3} C_{2}

MXene 表面的含氧基团在退火后没有完全去除。图 3 分别给出了 Ti 2 p、

Sn 3 d, S 2

p 和 C 1s 的高分辨率光谱。在锡 3d 光谱中，位于 495 eV 和 486.4 eV 的两个强特征峰分别归因于

{Sn}^{2 +} 3 d_{5 / 2}

和

{Sn}^{2 +} 3 d_{3 / 2}

。结果表明，杂化物中存在大量的 SnS 纳米粒子，这是

Fig. 2. a) X-ray diffraction spectra of

T_{3} C_{2}

MXene,

T_{3} C_{2}

MXene@C@SnS

_{2}, C @ S n S

and

T_{3} C_{2} M X e n e @ C @ S n S

. b) Raman spectrum of

T_{3} C_{2}

MXene@C@SnS.
图 2：a)

T_{3} C_{2}

MXene、

T_{3} C_{2}

MXene@C@SnS

_{2}, C @ S n S

和

T_{3} C_{2} M X e n e @ C @ S n S

的 X 射线衍射光谱；b)

T_{3} C_{2}

MXene@C@SnS 的拉曼光谱。

Fig. 3. High resolution spectrum of a) Sn 3 d , b)

Ti 2 p, c) S 2 p

, d) C 1 s .
图 3.a) Sn 3 d , b)

Ti 2 p, c) S 2 p

, d) C 1 s 的高分辨率光谱。
caused by the reduction of

{Sn}^{4 +}

during annealing. The two weak peaks at 495.7 and 487.2 eV are assigned to

{Sn}^{4 +} 3 d_{5 / 2}

and

{Sn}^{4 +} 3 d_{3 / 2}

, respectively

[25, 26]

. The phenomenon can be explained by that a little insufficient carbon source inside the intermediate

{SnS}_{2}

nanoparticles cause that

{SnS}_{2}

cannot be completely reduced to SnS during annealing. In Ti 2 p spectrum, the two characteristic peaks at 463.9 and 458.7 eV can be assigned to the

{Ti}^{3 +} 2 p_{1 / 2}

and

{Ti}^{3 +} 2 p_{3 / 2}

{Ti}_{3} C_{2}

MXene [27]. In addition, the typical peaks at 464.8 and 459.5 eV are attributed to the

2 p_{1 / 2}

and

2 p_{3 / 2}

orbitals of the Ti-O bond, which are derived from the hydroxyl

(- OH)

anchored on the surface of the

{Ti}_{3} C_{2}

MXene. In S 2 p spectrum, the peaks at 163.1 and 161.49 eV correspond to the

S^{2 -} 2 p_{1 / 2}

and

S^{2 -} 2 p_{3 / 2}

orbitals of divalent sulfide. The additional peaks at 164.8 and 163.98 eV are assigned to

C - S

bonds and C-S-C bonds formed by covalent bonding of sulfur and carbon nanolayers. Additionally, the characteristic peak at 285.79 eV in the C 1s spectrum also confirms the presence of the C-S bond

[28, 29]

. The result indicates that SnS nanoparticles are chemically bonded to carbon nanolayers. A typical

C - C / C = C

peak at 284.8 eV confirms existence of graphitized carbon layer [30,31]. Compared with weak physical bonds, the robust C-S bond provides strong interface coupling between SnS and carbon nanolayers, which can significantly enhance the bonding force between SnS and carbon nanolayers and promote the electron transfer.
是由于退火过程中

{Sn}^{4 +}

的减少造成的。在 495.7 和 487.2 eV 处的两个弱峰分别归属于

{Sn}^{4 +} 3 d_{5 / 2}

和

{Sn}^{4 +} 3 d_{3 / 2}

以及

[25, 26]

。这种现象可以解释为中间的

{SnS}_{2}

纳米粒子内部的碳源稍有不足，导致

{SnS}_{2}

在退火过程中不能完全还原成 SnS。在 Ti 2 p 光谱中，463.9 和 458.7 eV 处的两个特征峰可归属于

{Ti}_{3} C_{2}

MXene 的

{Ti}^{3 +} 2 p_{1 / 2}

和

{Ti}^{3 +} 2 p_{3 / 2}

[27]。此外，464.8 和 459.5 eV 处的典型峰值归因于 Ti-O 键的

2 p_{1 / 2}

和

2 p_{3 / 2}

轨道，它们来自锚定在

{Ti}_{3} C_{2}

MXene 表面的羟基

(- OH)

。在 S 2 p 谱中，163.1 和 161.49 eV 处的峰分别对应于二价硫化物的

S^{2 -} 2 p_{1 / 2}

和

S^{2 -} 2 p_{3 / 2}

轨道。另外，164.8 和 163.98 eV 处的峰分别是

C - S

键和硫与碳纳米层共价键形成的 C-S-C 键。此外，C 1s 光谱中 285.79 eV 处的特征峰也证实了

[28, 29]

C-S 键的存在。这一结果表明，SnS 纳米粒子与碳纳米层发生了化学键合。284.8 eV 处的典型

C - C / C = C

峰证实了石墨化碳层的存在 [30,31]。与薄弱的物理键相比，坚固的 C-S 键在 SnS 和碳纳米层之间提供了强大的界面耦合，可以显著增强 SnS 和碳纳米层之间的结合力，促进电子转移。

Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) techniques were carried out to investigate morphology of

{Ti}_{3} {AlC}_{2}, {Ti}_{3} C_{2}

, C@SnS and

{Ti}_{3} C_{2}

MXene@C@SnS hybrids and the results are presented in Fig. 4 and Figs. S3 and 4. The pristine

{Ti}_{3} {AlC}_{2}

MAX phase shows a typical densely packed multilayer structure in micrometer size (Figs. S3a and b). The

{Ti}_{3} C_{2}

MXene exhibits a typical multilayer nanostructure after

{Ti}_{3} {AlC}_{2}

is etched by HF as shown in Figs. S3c and d. Fig. 4a shows that a large number of

{SnS}_{2}

nanoparticles are grown on the surface and in the interlayer of

{Ti}_{3} C_{2}

MXene to form layered rock stratum structure in

{Ti}_{3} C_{2}

MXene@C@SnS 2 hybrids due to the electrostatic adsorption of

{Sn}^{4 +}

on the surface and in the interlayer of multi-layer

{Ti}_{3} C_{2}

MXene

({Ti}_{3} C_{2}

MXene with polar groups on the surface after etching). The formation of

{SnS}_{2}

nanoparticles in

{Ti}_{3} C_{2}

MXene interlayer and on the surface of

{Ti}_{3} C_{2}

MXene is attributed to Oswald ripening. As a seed,

{Ti}_{3} C_{2}

MXene acts as a large crystal in reaction system and promotes the formation of uniform

{SnS}_{2}

nanoparticles on the surface and in the interlayer of

{Ti}_{3} C_{2}

MXene during hydrothermal treatment. SEM images of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids are illustrated in Fig. 4b and c. It is obvious that

{Ti}_{3} C_{2}

MXene@C@SnS still retains the layered rock structure after

{SnS}_{2}

is reduced to SnS through annealing. The SnS nanoparticles are evenly grown in the interlayer and on the surface of the multilayered

{Ti}_{3} C_{2}

MXene. TEM images reveal that a large number of SnS nanoparticles are grown uniformly on

{Ti}_{3} C_{2} MXene

(orange circles) and numerous nanopores exist in the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids (orange arrows) as shown in Fig. 4d. The unique porous structure possesses abundant ion transportation tunnels, which can supply a large number of intercalation sites for lithium ions and accelerate rapid transfer of electrolyte by shortening the electrolyte diffusion path. HRTEM images of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids (Fig. 4e) clearly reveals that the crystal lattice of SnS and an interplanar spacing of 0.29 nm can be observed, which is attributed to the typical (101) crystal plane of SnS [21,32]. In addition, an amorphous structure of the edge portion represents a thin layer of carbon. The SEAD pattern (Fig. S5) shows that distinct diffraction rings related to (101) plane of SnS . The weak diffraction ring and distinct spot indicate a high degree of crystallinity from SnS in the hybrids. The energy dispersive disperse X-ray (EDX) elemental mapping images (Fig. 4 g ) clearly reveals that

Sn, S, Ti

and C elements are uniformly distributed in the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids.
扫描电子显微镜（SEM）和高分辨率透射电子显微镜（HR-TEM）技术用于研究

{Ti}_{3} {AlC}_{2}, {Ti}_{3} C_{2}

、C@SnS 和

{Ti}_{3} C_{2}

MXene@C@SnS 混合物的形态，结果见图 4 和图 S3 和图 4。原始的

{Ti}_{3} {AlC}_{2}

MAX 相显示出典型的致密多层结构，尺寸为微米级（图 S3a 和 b）。如图 S3c 和 d 所示，

{Ti}_{3} C_{2}

MXene 在

{Ti}_{3} {AlC}_{2}

被高频蚀刻后呈现出典型的多层纳米结构。图 4a 显示，在

{Ti}_{3} C_{2}

MXene@C@SnS 2 杂化物中，由于

{Sn}^{4 +}

在

{Ti}_{3} C_{2}

MXene 表面和多层

({Ti}_{3} C_{2}

MXene 层间的静电吸附（蚀刻后），大量

{SnS}_{2}

纳米颗粒在

{Ti}_{3} C_{2}

MXene 的表面和层间生长，形成层状岩层结构。

{SnS}_{2}

纳米粒子在

{Ti}_{3} C_{2}

MXene 层间和

{Ti}_{3} C_{2}

MXene 表面的形成归因于奥斯瓦尔德熟化。作为种子，

{Ti}_{3} C_{2}

MXene 在反应体系中起着大晶体的作用，在水热处理过程中促进了

{SnS}_{2}

MXene 表面和夹层中均匀的

{SnS}_{2}

纳米粒子的形成。图 4b 和 c 展示了

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的扫描电镜图像。很明显，

{Ti}_{3} C_{2}

MXene@C@SnS 在

{SnS}_{2}

通过退火还原为 SnS 后仍保留了层状岩石结构。SnS 纳米颗粒均匀地生长在多层

{Ti}_{3} C_{2}

MXene 的层间和表面。 TEM 图像显示，如图 4d 所示，

{Ti}_{3} C_{2} MXene

上均匀生长着大量 SnS 纳米颗粒（橙色圆圈），

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物中存在大量纳米孔（橙色箭头）。这种独特的多孔结构具有丰富的离子传输通道，可为锂离子提供大量的插层位点，并通过缩短电解质扩散路径加速电解质的快速转移。

{Ti}_{3} C_{2}

MXene@C@SnS杂化物的 HRTEM 图像（图 4e）清楚地显示出 SnS 的晶格和 0.29 nm 的平面间距，这归因于 SnS 的典型（101）晶面[21,32]。此外，边缘部分的无定形结构代表了一薄层碳。SEAD 图样（图 S5）显示了与 SnS 的（101）晶面有关的明显衍射环。微弱的衍射环和明显的斑点表明混合物中 SnS 的结晶度很高。能量色散 X 射线（EDX）元素图谱（图 4 g）清楚地显示，

Sn, S, Ti

和 C 元素均匀地分布在

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物中。

To investigate the porosity and surface area of the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids, the nitrogen adsorption-desorption was tested and the result is shown in Fig. 4f. A significant hysteresis loop appears in the medium pressure region of the adsorption-desorption isotherms,
为了研究

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的孔隙率和表面积，测试了氮的吸附-解吸情况，结果如图 4f 所示。在吸附-解吸等温线的中压区出现了明显的滞后环、

Fig. 4. a) SEM images of a)

{Ti}_{3} C_{2}

MXene@C@SnS

_{2}

and b, c)

{Ti}_{3} C_{2}

MXene@C@SnS at different magnifications. d) TEM images of the Ti

{Ti}_{3} C_{2}

MXene@C@SnS. e) HRTEM image of

{Ti}_{3} C_{2}

MXene@C@SnS. f) Nitrogen adsorption-desorption isotherm curves and the pore size distribution curves (illustration) of

{Ti}_{3} C_{2} MXene

@C@SnS hybrids. g) Elemental mapping images of

{Ti}_{3} C_{2}

MXene@C@SnS.
图 4. a) 不同放大倍数下 a)

{Ti}_{3} C_{2}

MXene@C@SnS

_{2}

和 b, c)

{Ti}_{3} C_{2}

MXene@C@SnS 的扫描电镜图像。 d) Ti

{Ti}_{3} C_{2}

MXene@C@SnS 的 TEM 图像。f)

{Ti}_{3} C_{2} MXene

@C@SnS 混合物的氮吸附-解吸等温线曲线和孔径分布曲线（插图）。 g)

{Ti}_{3} C_{2}

MXene@C@SnS 的元素图谱图像。
indicating the presence of a number of small nanopores. The corresponding Brunauer-Emmett-Teller (BET) specific surface area of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids is calculated to be

255.78 m^{2} g^{- 1}

and the large specific surface area can provide multitudinous reactive sites for lithiation/de-lithiation reaction by increasing the contact area between electrolyte and active material. In addition, the corresponding pore size distribution curves (illustration in Fig. 4 f ) shows that the pore size is ranged from 1.80 to 21.16 nm and concentrated at 2.64 nm . A large number of nanopores in the

{Ti}_{3} C_{2}

MXene interlayer can create plenty of lithium ion channels to promote free transfer of lithium ions and shorten the lithium ion transport path. Numerous SnS nanoparticles in

{Ti}_{3} C_{2}

MXene interlayer in

{Ti}_{3} C_{2}

MXene@C@SnS hybrids can cause a large number of nanopores in the

{Ti}_{3} C_{2} MXene

interlayer to provide a large specific surface area. In addition, to further explore the specific surface area contribution of SnS nanoparticles in hybrids, the BET specific surface area of C@SnS was also tested, and the corresponding specific surface area of C@SnS is

252.98 m^{2} / g

as shown in Fig. S6, which shows that C@SnS has a large specific surface area. This result also demonstrates that

{Ti}_{3} C_{2}

MXene@C@SnS hybrids has a large specific surface area because a large number of nanopores in the

{Ti}_{3} C_{2}

MXene interlayer and SnS nanoparticles contribute to more specific surface area to the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids.
表明存在许多小纳米孔。根据计算，

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物相应的布鲁诺-艾美特-泰勒（BET）比表面积为

255.78 m^{2} g^{- 1}

，大的比表面积可以通过增加电解质和活性材料之间的接触面积，为锂化/脱锂反应提供大量反应位点。此外，相应的孔径分布曲线（如图 4 f 所示）表明，孔径范围在 1.80 至 21.16 nm 之间，并集中在 2.64 nm。

{Ti}_{3} C_{2}

MXene夹层中大量的纳米孔可以形成大量的锂离子通道，促进锂离子的自由传输，缩短锂离子的传输路径。在

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物中，

{Ti}_{3} C_{2}

MXene 夹层中的大量 SnS 纳米粒子可以在

{Ti}_{3} C_{2} MXene

夹层中形成大量纳米孔，从而提供较大的比表面积。此外，为了进一步探讨杂化物中 SnS 纳米粒子的比表面积贡献，还测试了 C@SnS 的 BET 比表面积，如图 S6 所示，C@SnS 的相应比表面积为

252.98 m^{2} / g

，这表明 C@SnS 具有较大的比表面积。这一结果也证明了

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物具有较大的比表面积，因为

{Ti}_{3} C_{2}

MXene 夹层和 SnS 纳米粒子中的大量纳米孔有助于增加

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的比表面积。

To estimate the electrochemical performance of

{Ti}_{3} C_{2} MXe

ne@C@SnS hybrids as lithium-ion battery anodes, the

{Ti}_{3} C_{2}

MXene@C@SnS electrode was prepared and assembled into CR2032-type button cells with lithium wafer as counter electrode. The lithium-ion storage mechanism of the

{Ti}_{3} C_{2}

MXene@C@SnS electrode were first evaluated by cyclic voltammetry at a cut-off voltage window of

0.01 - 3.0 V

with a sweep rates of

0.1 mV s^{- 1}

as shown in Fig. 5a. In the first cathodic scan (first lithiation), the three peaks at 1.84, 1.17 and 0.08 V due to the SnS lithiation. In detail, the peak at 1.84 V is attributed to the formation of

{Li}_{x} SnS

{Li}^{+}

insertion into the SnS . The strong peak at 1.17 V is attributed to the phase inversion reaction of

{Li}_{x} SnS

to metallic Sn and

{Li}_{2} S

. The broad peak at

0.08 - 0.5 V

is attributed to metal Sn and lithium ion alloying reaction to form

{Li}_{x} Sn

[33-35]. In the first anodic scan (first de-lithiation), two peaks at 0.5 and 0.65 V are ascribed to the multi-step dealloying reaction of

{Li}_{x} Sn

Sn [11, 36, 37]

. The broad peak at 1.3 V is attributed to the oxidation of Sn to

SnS [38, 39]

, and the strong peak at 1.9 V is attributed to a small amount of SnS further converted to

{SnS}_{2} [6, 40]

. Additionally, a broad weak peak at 2.38 V is resulted from the conversion reaction of

{Li}_{2} S

to elemental S [41]. During the subsequent scans, two cathode peaks at 1.84 and 1.17 V disappear while a newly distinct cathodic peak at 1.2 V is attributed to the lithiation reaction of SnS . The peak at 1.9 V is weakened at following scans due to consumption of

{SnS}_{2}

during cycling. CV curves become gradually stable and be nearly overlapped in subsequent scans. The result indicates that

{Ti}_{3} C_{2}

MXene@C@SnS electrode shows the excellent cycling stability during charging and discharging.
为了评估

{Ti}_{3} C_{2} MXe

ne@C@SnS 混合材料作为锂离子电池阳极的电化学性能，制备了

{Ti}_{3} C_{2}

MXene@C@SnS 电极，并将其组装到以锂晶片为对电极的 CR2032 型纽扣电池中。如图 5a 所示，首先通过循环伏安法评估了

{Ti}_{3} C_{2}

MXene@C@SnS 电极的锂离子存储机制，截止电压窗口为

0.01 - 3.0 V

，扫描速率为

0.1 mV s^{- 1}

。在第一次阴极扫描（第一次光化）中，由于 SnS 光化，在 1.84、1.17 和 0.08 V 处出现了三个峰值。具体来说，1.84 V 处的峰值是由于

{Li}_{x} SnS

通过

{Li}^{+}

插入到 SnS 中形成的。1.17 V 处的强峰归因于

{Li}_{x} SnS

与金属锡和

{Li}_{2} S

的相反转反应。

0.08 - 0.5 V

处的宽峰是金属锡和锂离子合金化反应形成

{Li}_{x} Sn

的结果 [33-35]。在第一次阳极扫描（第一次脱锂）中，0.5 V 和 0.65 V 处的两个峰是

{Li}_{x} Sn

与

Sn [11, 36, 37]

的多步脱锂反应所致。1.3 V 处的宽峰归因于锡氧化成

SnS [38, 39]

，1.9 V 处的强峰归因于少量的 SnS 进一步转化成

{SnS}_{2} [6, 40]

。此外，

{Li}_{2} S

向元素 S 的转化反应产生了 2.38 V 的宽弱峰[41]。在随后的扫描过程中，1.84 V 和 1.17 V 的两个阴极峰消失了，而 1.2 V 新出现的明显阴极峰则是由于 SnS 的锂化反应。由于在循环过程中消耗了

{SnS}_{2}

，1.9 V 处的峰值在随后的扫描中减弱。CV 曲线逐渐变得稳定，并在随后的扫描中几乎重叠。结果表明，

{Ti}_{3} C_{2}

MXene@C@SnS 电极在充放电过程中表现出优异的循环稳定性。

In order to evaluate the specific capacity of the

{Ti}_{3} C_{2} - MXe

ne@C@SnS hybrids as anode materials, galvanostatic charge and discharge tests were carried out and the results are illustrated in Fig. 5b. The galvanostatic charge and discharge curves of

{Ti}_{3} C_{2}

MXene@C@SnS
为了评估作为阳极材料的

{Ti}_{3} C_{2} - MXe

ne@C@SnS 杂化物的比容量，进行了电致静电充电和放电测试，结果如图 5b 所示。图 5b 中显示了

{Ti}_{3} C_{2}

MXene@C@SnS 混合物的静电充电和放电曲线。

Fig. 5. a)

C V

curves of

{Ti}_{3} C_{2}

MXene@C@SnS electrode at scan rate of

0.1 mV s^{- 1}

. b) The first three galvanostatic charge and discharge curves of

{Ti}_{3} C_{2}

MXene@C@SnS electrode at a current of

0.1 A g^{- 1}

. c) Rate performance of

{Ti}_{3} C_{2}

MXene@C@SnS and C@SnS electrodes at different current densities. d) Charge and discharge curves of

{Ti}_{3} C_{2}

MXene@C@SnS electrodes at different current densities. e) Long-term cycling stability of

{Ti}_{3} C_{2}

MXene@C@SnS electrode at

1 {Ag}^{- 1}

. f) Mechanism diagram of electrolyte transfer during charging and discharging of

{Ti}_{3} C_{2}

MXene@C@SnS electrode.
图 5. a)

{Ti}_{3} C_{2}

MXene@C@SnS 电极在扫描速率为

0.1 mV s^{- 1}

时的

C V

曲线。 b)

{Ti}_{3} C_{2}

MXene@C@SnS 电极在电流为

0.1 A g^{- 1}

时的前三条电致静电充放电曲线。 c)

{Ti}_{3} C_{2}

MXene@C@SnS 和 C@SnS 电极在不同电流密度下的速率性能。d)

{Ti}_{3} C_{2}

MXene@C@SnS 电极在不同电流密度下的充放电曲线。 e)

{Ti}_{3} C_{2}

MXene@C@SnS 电极在

1 {Ag}^{- 1}

下的长期循环稳定性。 f)

{Ti}_{3} C_{2}

MXene@C@SnS 电极充放电过程中电解液转移的机理图。
electrode are obtained by using a current density of

0.1 A g^{- 1}

for charging/discharging with a voltage range of

0.01 - 3.0 V

. In the first charge and discharge, the

{Ti}_{3} C_{2}

MXene@C@SnS electrode exhibits a remarkable initial discharge capacity and charge capacity of 2133 and

1491 mA h^{- 1}

, respectively. Additionally, the initial Coulombic efficiency (CE) is

69.9 % .30 .1 %

irreversible capacity loss at the first cycle is attributed to the formation of inorganic solid electrolyte interphase (SEI) film [42], decomposition of electrolyte [43], irreversible generation of a few of

{Li}_{x} Sn

alloys. Besides, lithium-ions can not be extracted from SnS lattice completely. In the following two discharge/charge cycles, the corresponding specific capacities are 1473/1435 and 1464/1432 mA h

g^{- 1}

, respectively. The Coulombic efficiency rapidly increases to

97.4 %

and

97.8 %

in 2 nd and 3rd cycles, respectively. The result shows that the

{Ti}_{3} C_{2}

MXene@C@SnS electrode exhibits excellent reversibility capacity of the lithium-ion storage at a low current density.
在

0.01 - 3.0 V

的电压范围内，以

0.1 A g^{- 1}

的电流密度进行充电/放电，可获得

{Ti}_{3} C_{2}

MXene@C@SnS 电极的初始放电容量和充电容量分别为 2133 和

1491 mA h^{- 1}

。在第一次充放电中，

{Ti}_{3} C_{2}

MXene@C@SnS 电极表现出显著的初始放电容量和充电容量，分别为 2133 和

1491 mA h^{- 1}

。此外，初始库仑效率（CE）为

69.9 % .30 .1 %

，第一个循环的不可逆容量损失归因于无机固体电解质相间（SEI）膜的形成[42]、电解质的分解[43]、少量

{Li}_{x} Sn

合金的不可逆生成。此外，锂离子无法完全从 SnS 晶格中提取出来。在随后的两个放电/充电循环中，相应的比容量分别为 1473/1435 和 1464/1432 mA h

g^{- 1}

。库仑效率在第 2 次和第 3 次循环中分别迅速增加到

97.4 %

和

97.8 %

。结果表明，

{Ti}_{3} C_{2}

MXene@C@SnS 电极在低电流密度下具有出色的锂离子存储可逆容量。
The rate performance is vital for estimating properties of anode materials and the

{Ti}_{3} C_{2}

MXene@C@SnS electrode deliveries an outstanding rate capability as shown in Fig. 5c. For comparison, the rate performances of C@SnS electrode was also studied. The C@SnS electrode delivers a reversible capacities of

846, 617, 513, 388, 393, 334

, 292 and

659 mA h g^{- 1}

0.1, 0.3, 0.5, 1, 2, 3, 5 A g^{- 1}

and restore to 0.1

A g^{- 1}

, respectively (Fig. 5c). In contrast,

{Ti}_{3} C_{2}

MXene@C@SnS hybrids as anode material of lithium ion batteries exhibit superior electrochemical performance. The

{Ti}_{3} C_{2}

MXene@C@SnS electrode delivers an excellent reversible specific capacity of

1473, 1415, 1213, 1048, 869

and

708 mA h g^{- 1}

at different current densities of

0.1, 0.3, 0.5, 1, 2

and

3 A g^{- 1}

, respectively. In addition, the electrode can still deliver a high reversible capacity of

640 mA h^{- 1}

when the current density reaches to

5 A g^{- 1}

. Compared with the original capacity, the electrode still maintains an excellent discharge specific capacity of

1417 mA h g^{- 1}

with
如图 5c 所示，

{Ti}_{3} C_{2}

MXene@C@SnS 电极具有出色的速率性能。为了进行比较，还研究了 C@SnS 电极的速率性能。C@SnS 电极在

0.1, 0.3, 0.5, 1, 2, 3, 5 A g^{- 1}

和恢复到 0.1

A g^{- 1}

时的可逆容量分别为

846, 617, 513, 388, 393, 334

、292 和

659 mA h g^{- 1}

（图 5c）。相比之下，作为锂离子电池负极材料的

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物则表现出优异的电化学性能。在

0.1, 0.3, 0.5, 1, 2

和

3 A g^{- 1}

的不同电流密度下，

{Ti}_{3} C_{2}

MXene@C@SnS 电极的可逆比容量分别为

1473, 1415, 1213, 1048, 869

和

708 mA h g^{- 1}

，表现出色。此外，当电流密度达到

5 A g^{- 1}

时，电极仍能提供

640 mA h^{- 1}

的高可逆容量。与原来的容量相比，该电极仍能保持

1417 mA h g^{- 1}

的出色放电比容量。
little decay when the currents density returns to

0.1 A g^{- 1}

. Corresponding charge-discharge curves at each current density are illustrated in Fig. 5 d . The improvement of remarkably capacity of

{Ti}_{3} C_{2}

MXene@C@SnS electrode is resulted from the porous robust layered rock stratum structure, which can promote the rapid transfer of lithium ions, accommodate a large amount of lithium ion insertion and extraction and possess good conductivity provided by the carbon nanolayer. Furthermore, the cycle stability test of the electrode was continually performed at a current density of

0.5 A g^{- 1}

over 70 cycles after the 80 -cycle rate performance test. The

{Ti}_{3} C_{2}

MXene@C@SnS electrode still delivers an excellent reversible capacity of

1252 mA h g^{- 1}

when the current density returns to

0.5 A g^{- 1}

. The electrode exhibits a high reversible capacity of

1204.8 mA h g^{- 1}

at the 112 nd cycle and even maintains a reversible capacity of

1142.2 mA h g^{- 1}

for 150 cycles, which indicates that the

{Ti}_{3} C_{2}

MXene@C@SnS electrode possesses excellent cycle stability and the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids maintain a superior stability during the rapid deintercalation of lithium ions.
当电流密度恢复到

0.1 A g^{- 1}

时，衰减很小。图 5 d 显示了各电流密度下相应的充放电曲线。

{Ti}_{3} C_{2}

MXene@C@SnS 电极容量的显著提高得益于多孔坚固的层状岩层结构，这种结构能够促进锂离子的快速转移，容纳大量的锂离子插入和提取，并且碳纳米层具有良好的导电性。此外，在进行了 80 次循环速率性能测试后，又以

0.5 A g^{- 1}

的电流密度持续进行了 70 次循环稳定性测试。当电流密度恢复到

0.5 A g^{- 1}

时，

{Ti}_{3} C_{2}

MXene@C@SnS 电极仍能提供

1252 mA h g^{- 1}

的出色可逆容量。该电极在第 112 次循环时显示出

1204.8 mA h g^{- 1}

的高可逆容量，甚至在 150 次循环时仍能保持

1142.2 mA h g^{- 1}

的可逆容量，这表明

{Ti}_{3} C_{2}

MXene@C@SnS 电极具有出色的循环稳定性，而

{Ti}_{3} C_{2}

MXene@C@SnS 混合材料在锂离子快速脱插过程中也能保持出色的稳定性。

The long-term cycling stability of

{Ti}_{3} C_{2}

MXene@C@SnS electrode and C@SnS electrode is further investigated at a high current density of

1 A g^{- 1}

as shown in Fig. 5e. The C@SnS electrode exhibits an initial discharge specific capacity of

1145 mA h g^{- 1}

and corresponding Coulombic efficiency is

62.7 %

. The electrode exhibits inferior cycling stability and the capacity is gradually decreased to

272 mA h^{- 1}

for 350 cycles at

1 A g^{- 1}

. Especially, the

{Ti}_{3} C_{2}

MXene@C@SnS electrode maintains an excellent reversible capacity of

1050 mA h g^{- 1}

over 350 cycles at

1 A g^{- 1}

. The first Coulombic efficiency is

49.9 %

but it increases rapidly from the second cycle. The Coulombic efficiency retains over

99 %

and basically stable at the seventh cycle because SEI film is already fully formed. The lithium-ion insertion and extraction have been steady after first cycle. At the charge/discharge cycles, the capacity initially decreases to

918 mA h g^{- 1}

, afterwards, the capacity gradually increases to

1050 mA h g^{- 1}

and tends to be steady. The electrode can reach a high specific capacity of

1126 mA h g^{- 1}

even at the 185th cycle. The capacity increases with the rise of the cycles resulted from the formation of organic polymeric/gel layers and active material is activated during cycling

[12, 17, 28]

. The cycling stability of

{Ti}_{3} C_{2}

MXene@C@SnS electrode is superior to previously reported tin-based materials (Table S1) due to robust layered rock stratum structure. The increase of cycleability of

{Ti}_{3} C_{2}

MXene@C@SnS electrode is resulted from the robust porous layered rock stratum structure, which can relieve the volume fluctuation during rapid insertion/extraction of lithium ions.
如图 5e 所示，进一步研究了

{Ti}_{3} C_{2}

MXene@C@SnS 电极和 C@SnS 电极在

1 A g^{- 1}

高电流密度下的长期循环稳定性。C@SnS 电极的初始放电比容量为

1145 mA h g^{- 1}

，相应的库仑效率为

62.7 %

。该电极的循环稳定性较差，在

1 A g^{- 1}

条件下循环 350 次，容量逐渐下降到

272 mA h^{- 1}

。尤其是

{Ti}_{3} C_{2}

MXene@C@SnS 电极，在

1 A g^{- 1}

条件下循环 350 次，仍能保持

1050 mA h g^{- 1}

的出色可逆容量。第一个库仑效率为

49.9 %

，但从第二个循环开始迅速增加。库仑效率保持在

99 %

以上，并且在第七个循环时基本稳定，因为 SEI 膜已经完全形成。锂离子的插入和提取在第一个循环后保持稳定。在充放电循环中，容量最初下降到

918 mA h g^{- 1}

，之后容量逐渐上升到

1050 mA h g^{- 1}

并趋于稳定。即使在第 185 个循环时，电极也能达到

1126 mA h g^{- 1}

的高比容量。容量随着循环次数的增加而增加，这是由于有机聚合物/凝胶体层的形成和活性材料在循环

[12, 17, 28]

过程中被激活。

{Ti}_{3} C_{2}

MXene@C@SnS电极的循环稳定性优于之前报道的锡基材料（表 S1），这是由于其具有坚固的分层岩层结构。

{Ti}_{3} C_{2}

MXene@C@SnS电极循环稳定性的提高得益于其坚固的多孔层状岩层结构，这种结构可以缓解锂离子快速插入/萃取过程中的体积波动。

The mechanism diagram of electrolyte transference during charging and discharging of

{Ti}_{3} C_{2}

MXene@C@SnS electrode is presented in Fig. 5f. The designed

{Ti}_{3} C_{2}

MXene@C@SnS hybrids exhibit excellent electrochemical properties including high reversible capacity, remarkable rate performance and outstanding long-term cycling stability at high currents, which can be ascribed to the following merits: (1) The SnS nanoparticles are uniformly anchored on the surface and in the interlayer of the multilayer

{Ti}_{3} C_{2}

MXene, which can effectively prevent the capacity from attenuation caused by the aggregation of the SnS nanoparticles during rapid deintercalation of lithium ions. (2) The layered rock stratum structure of

{Ti}_{3} C_{2}

MXene@C@SnS hybrids effectively alleviate the large volume fluctuation caused by the rapid insertion and extraction of lithium ions, ensuring the stability of the structure during rapid charge and discharge, thereby improving long-cycle stability. (3)

{Ti}_{3} C_{2}

MXene@C@SnS with layered rock stratum structure has many nanopores to shorten the transfer distance of lithium ions to promote the rapid transfer of lithium ions. In addition, the porous structure enlarges the contact area of the electrolyte to provide abundant active sites for deintercalation of lithium, thereby significantly increasing the high capacity and excellent rate capability. (4) The carbon nanolayer in

{Ti}_{3} C_{2}

MXene@C@SnS hybrids can significantly improve electron/ion conductivity, thereby improving the rate performance. As a result,

{Ti}_{3} C_{2}

MXene@C@SnS electrode exhibits excellent electrochemical performance.
图 5f 展示了

{Ti}_{3} C_{2}

MXene@C@SnS 电极充放电过程中电解质转移的机理图。所设计的

{Ti}_{3} C_{2}

MXene@C@SnS 混合物具有优异的电化学性能，包括高可逆容量、显著的速率性能和大电流下出色的长期循环稳定性：(1) SnS 纳米粒子均匀地锚定在多层

{Ti}_{3} C_{2}

MXene 的表面和层间，可有效防止锂离子快速脱插时 SnS 纳米粒子聚集导致的容量衰减。(2)

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物的分层岩层结构可有效缓解锂离子快速插入和提取时引起的较大体积波动，确保快速充放电时结构的稳定性，从而提高长周期稳定性。(3)

{Ti}_{3} C_{2}

MXene@C@SnS 具有层状岩层结构，具有许多纳米孔，缩短了锂离子的传输距离，促进了锂离子的快速传输。此外，多孔结构扩大了电解液的接触面积，为锂的脱嵌提供了丰富的活性位点，从而显著提高了高容量和优异的速率能力。(4)

{Ti}_{3} C_{2}

MXene@C@SnS 杂化物中的碳纳米层可显著提高电子/离子传导性，从而改善速率性能。因此，

{Ti}_{3} C_{2}

MXene@C@SnS 电极具有优异的电化学性能。

To further explain the improvement of electrochemical performance of the

{Ti}_{3} C_{2}

MXene@C@SnS electrode, electrochemical impedance spectroscopy (EIS) was conducted to evaluate internal resistance of

{Ti}_{3} C_{2}

MXene@C@SnS and C@SnS electrodes as illustrated in Fig. 6a. The Nyquist plots of the impedance spectrum are composed of a semicircle in high frequency region and a slanted straight line in the low frequency region. The formation of a semicircle is related to the electron transfer resistance at the interface of the electrolyte/electrode. The slanted straight line at low frequency correspond to the lithium-ion diffusion in the electrode material and the high slope indicates the rapid diffusion of lithium ions in transportation tunnels [44]. In the EIS spectrum, the extremely sloped straight line at low frequency region is related to the faster lithium ion diffusion kinetics of the two electrodes, which is resulted from nanoporosity of layered rock stratum structure. Additionally, a smaller diameter semicircle of

{Ti}_{3} C_{2}

MXene@C@SnS electrode appear in high frequency and the results show that

{Ti}_{3} C_{2}

MXene@C@SnS has a smaller charge transfer resistance (

R_{ct} = 79 Ω

) than

SnS @ C

electrode

(R_{ct} = 120 Ω)

. The faster lithium-ion diffusion kinetics and smaller charge transfer resistance facilitate lithium-ions insertion and extraction in the

{Ti}_{3} C_{2}

MXene@C@SnS electrode. Meanwhile, the diffusion coefficient of lithium ions is further calculated by Nyquist plots of oblique lines in the low frequency region. Fig. 6b presents the relationship between the Impedance (

Z_{r}

) and the inverse of the square root of the angular frequency (

ω^{- 1 / 2}

) of

{Ti}_{3} C_{2}

MXene@C@SnS and C@SnS electrodes in low frequency region. By fitting points, the slope of the line is obtained. The lithium ions diffusion coeffcient

(D_{Li})

is calculated (the detailed calculation process is shown in the supporting information). Both the

{Ti}_{3} C_{2}

MXene@C@SnS and C@SnS electrodes exhibit high lithium ions diffusion coefficients and the

D_{Li}

value of in the MXene@C@SnS electrode (

6.45 \times 10^{- 12}

) is higher than that in the

C @ S n S

electrode

(4.69 \times 10^{- 12})

. The large lithium ions diffusion coefficient (

D_{Li}

) of

{Ti}_{3} C_{2}

MXene@C@SnS electrodes is resulted from the layered rock stratum structure which has many nanopores to provide rich ion transport tunnels. In addition, large specific surface area increases the reactive sites of the electrolyte and the active material, which endow

{Ti}_{3} C_{2}

MXene@C@SnS hybrids with extraordinary rate performance

[45, 46]

.
为了进一步解释

{Ti}_{3} C_{2}

MXene@C@SnS 电极电化学性能的改善，对

{Ti}_{3} C_{2}

MXene@C@SnS 和 C@SnS 电极的内阻进行了电化学阻抗谱（EIS）评估，如图 6a 所示。阻抗谱的奈奎斯特图由高频区的半圆和低频区的斜直线组成。半圆的形成与电解质/电极界面上的电子转移电阻有关。低频的斜直线与电极材料中的锂离子扩散相对应，高斜率表示锂离子在传输隧道中的快速扩散[44]。在 EIS 频谱中，低频区域的直线斜率极高，这与两个电极的锂离子扩散动力学速度较快有关，这是由层状岩层结构的纳米孔隙度造成的。此外，

{Ti}_{3} C_{2}

MXene@C@SnS 电极在高频出现了一个直径较小的半圆，结果表明

{Ti}_{3} C_{2}

MXene@C@SnS 的电荷转移电阻（

R_{ct} = 79 Ω

）小于

SnS @ C

电极

(R_{ct} = 120 Ω)

。较快的锂离子扩散动力学和较小的电荷转移电阻有利于锂离子在

{Ti}_{3} C_{2}

MXene@C@SnS 电极中的插入和提取。同时，锂离子的扩散系数是通过低频区斜线的奈奎斯特图进一步计算得出的。图 6b 显示了低频区

{Ti}_{3} C_{2}

MXene@C@SnS 和 C@SnS 电极的阻抗（

Z_{r}

）与角频率平方根的倒数（

ω^{- 1 / 2}

）之间的关系。通过拟合点，可以得到直线的斜率。计算出了锂离子扩散系数

(D_{Li})

（详细计算过程见辅助信息）。

{Ti}_{3} C_{2}

MXene@C@SnS 和 C@SnS 电极都表现出较高的锂离子扩散系数，并且 MXene@C@SnS 电极的

D_{Li}

值（

6.45 \times 10^{- 12}

）高于

C @ S n S

电极的

(4.69 \times 10^{- 12})

。

{Ti}_{3} C_{2}

MXene@C@SnS 电极具有较大的锂离子扩散系数（

D_{Li}

），这是因为层状岩层结构具有许多纳米孔，为离子传输提供了丰富的通道。此外，大的比表面积增加了电解质和活性材料的反应位点，从而使

{Ti}_{3} C_{2}

MXene@C@SnS 混合物具有非凡的速率性能

[45, 46]

。

To further understand the lithium-ion storage mechanism at various current densities, electrochemical reaction kinetics of the

{Ti}_{3} C_{2}

MXene@C@SnS electrode was evaluated as shown in Fig. 7. Fig. 7a shows that CV curves maintain similar shapes at sweep rates of

0.1 - 5 mV s^{- 1}

. In addition, as the sweep rate increases, the cathodic and anodic peaks transfer to lower and higher potentials, respectively. The result demonstrates that the

{Ti}_{3} C_{2}

MXene@C@SnS electrode keeps smaller polarization at different sweep rates [47]. In general, electrochemical behavior of lithium-ion storage is divided into surface capacitive behavior and diffusion-controlled insertion process. The peak current

(i)

vs sweep rate (

v

) observe the following relationships [48].
为进一步了解不同电流密度下的锂离子存储机制，对

{Ti}_{3} C_{2}

MXene@C@SnS 电极的电化学反应动力学进行了评估，如图 7 所示。图 7a 显示，当扫描速率为

0.1 - 5 mV s^{- 1}

时，CV 曲线保持相似的形状。此外，随着扫描速率的增加，阴极峰和阳极峰分别转移到较低和较高的电位。结果表明，

{Ti}_{3} C_{2}

MXene@C@SnS 电极在不同的扫描速率下保持较小的极化[47]。一般来说，锂离子存储的电化学行为分为表面电容行为和扩散控制的插入过程。峰值电流

(i)

与扫描速率 (

v

) 的关系如下 [48]。

i = a v^{b}

\log (i) = blog (v) + \log (a)

Herein, a and b are constants. The values of slope b are determined by electrochemical behavior. The diffusion-controlled insertion process is dominated by the electrochemical behavior when b value is close to 0.5 . The lithium-ion storage mechanism is dominated by surface capacitive behavior when b value is close to 1 . Herein, b values corresponding to the cathodic and anodic peaks are 0.78 and 0.74 (Fig. 7b), respectively, which are both close to 0.75 , indicating that both surface pseudocapacitance behavior and diffusion-controlled insertion process control the electrochemical behavior of the lithium-ion storage. The capacitive contribution proportion of the lithium-ion storage can be calculated by following formula [49].
其中，a 和 b 是常数。斜率 b 的值由电化学行为决定。当 b 值接近 0.5 时，扩散控制的插入过程由电化学行为主导。当 b 值接近 1 时，锂离子存储机制由表面电容行为主导。在这里，阴极峰和阳极峰对应的 b 值分别为 0.78 和 0.74（图 7b），都接近 0.75，表明表面伪电容行为和扩散控制的插入过程都控制着锂离子存储的电化学行为。锂离子储能的电容贡献比例可按下式计算[49]。

i = k_{1} v + k_{2} v^{0.5}

Where

k_{1}

and

k_{2}

are parameters at a given potential. The current

i

can be
其中

k_{1}

和

k_{2}

是给定电位下的参数。电流

i

可以是

Fig. 6. a) Electrochemical impedance spectra of

{Ti}_{3} C_{2}

MXene@C@SnS and C@SnS electrodes. b) Relationship between Impedance (

Z_{re}

) and the inverse of the square root of the angular frequency

(ω^{- 1 / 2})

of MXene@C@SnS and C@SnS electrodes in low frequency region.
图 6. a)

{Ti}_{3} C_{2}

MXene@C@SnS 和 C@SnS 电极的电化学阻抗谱。 b) MXene@C@SnS 和 C@SnS 电极在低频区的阻抗（

Z_{re}

）与角频率平方根的倒数

(ω^{- 1 / 2})

之间的关系。

Fig. 7. Kinetic analysis of

{Ti}_{3} C_{2}

MXene@C@SnS electrode in lithium-ion storage. a) CV curves at sweep rates of

0.1 - 5 mV s^{- 1}

. b) b values of log (peak current) vs.

\log

(sweep rate) plots at corresponding cathodic and anodic peaks ranging from 0.1 to

5 mV s^{- 1}

. c) CV curve and capacitance contribution (green area) to the lithiumion storage at a sweep rate of

1 mV s^{- 1}

. d) Capacitance contribution ratio under different sweep rates. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
图 7.a) 在扫频为

0.1 - 5 mV s^{- 1}

时的 CV 曲线。 b) 对数（峰值电流）与

\log

（扫频）图的 b 值在相应的阴极和阳极峰值范围从 0.c) CV 曲线和

1 mV s^{- 1}

扫频下锂离子存储的电容贡献（绿色区域）。(有关本图例中颜色的解释，请读者参阅本文的网络版）。
further assigned to surface capacitive behavior

(k_{1} v)

and diffusioncontrolled insertion process

(k_{2} v^{0.5})

. The percentage of pseudocapacitance contribution at the same potential can be obtained by formula (3) (pseudocapacitance

= k_{1} v / (k_{1} v + k_{2} v^{0.5})

. The total current responses of pseudocapacitance are calculated by the total current at same sweep rates [11]. The capacitance contribution of the

{Ti}_{3} C_{2}

MXene@C@SnS electrode is

52.36 %

at the sweep rate of

1 mV s^{- 1}

as shown in Fig. 7 c . The detailed capacitive contribution of the lithium-ion storage at various sweep rates is illustrated in Fig. 7 d . It is obvious that the capacitance contribution increases with rise of the sweep rate. The corresponding capacitance contributions are

29.33 %, 36.99 %, 48.14 %

52.36 %, 64.99 %

and

74.59 %

when the sweep rates are

0.1, 0.2, 0.5, 1, 2

and

5 mV s^{- 1}

, respectively. At low sweep rate, the lithium-ion storage is conducted by diffusion-controlled insertion process. Nevertheless, the capacitive behavior occupies the dominant status in lithium-ion storage at fast sweep rate. The capacitive lithium-ion storage at fast sweep rate endows

{Ti}_{3} C_{2}

MXene@C@SnS electrode with excellent rate performance and long-term cycling stability at high current density.
进一步归因于表面电容行为

(k_{1} v)

和扩散控制的插入过程

(k_{2} v^{0.5})

。相同电位下的伪电容贡献百分比可通过公式 (3) 得出（伪电容

= k_{1} v / (k_{1} v + k_{2} v^{0.5})

）。伪电容的总电流响应由相同扫描速率下的总电流计算得出 [11]。如图 7 c 所示，在扫频速率为

1 mV s^{- 1}

时，

{Ti}_{3} C_{2}

MXene@C@SnS 电极的电容贡献为

52.36 %

。图 7 d 详细说明了不同扫描速率下锂离子存储的电容贡献。很明显，电容贡献随扫描速率的增加而增加。当扫描速率为

0.1, 0.2, 0.5, 1, 2

和

5 mV s^{- 1}

时，相应的电容贡献分别为

29.33 %, 36.99 %, 48.14 %

、

52.36 %, 64.99 %

和

74.59 %

。在低扫频条件下，锂离子存储是通过扩散控制的插入过程进行的。然而，在快速扫描速率下，电容行为在锂离子存储中占据主导地位。快速扫描速率下的电容性锂离子存储赋予了

{Ti}_{3} C_{2}

MXene@C@SnS 电极优异的速率性能和高电流密度下的长期循环稳定性。

4. Conclusions 4.结论

The

{Ti}_{3} C_{2}

MXene@C@SnS hybrids with robust layered rock stratum

{Ti}_{3} C_{2}

具有坚固层状岩层的 MXene@C@SnS 混合物
structure were synthesized by uniformly in situ growth of SnS nanoparticles on the surface and in the interlayer of multilayer

{Ti}_{3} C_{2}

MXene, which is encapsulated by carbon nanolayers. The layered rock stratum structure with large specific surface area provides abundant reaction active sites for electrochemistry reaction and numerous nanopores by means of shortening electrolyte transport path to facilitate lithium-ion diffusion. The robust structure alleviate the pressure resulted from large volume fluctuations during rapid lithiation/de-lithiation. The

{Ti}_{3} C_{2}

MXene@C@SnS electrode delivers an extremely high reversible capacity of

1473 mA h^{- 1}

0.1 {Ag}^{- 1}

and exhibit a remarkable rate performance of reversible capacity

640 mA h g^{- 1}

5 {Ag}^{- 1}

. Additionally, the

{Ti}_{3} C_{2}

MXene@C@SnS electrode maintains an excellent cycle stability of ultrahigh capacity of

1050 mA h g^{- 1}

over 350 cycles at 1 A

g^{- 1}

. Kinetic analysis indicates that lithium-ion storage of the

{Ti}_{3} C_{2}

MXene@C@SnS electrode is dominated by surface pseudocapacitance behavior and diffusion controlled process. This study provides a new idea to prepare composite materials with porous layered rock stratum structure and the

{Ti}_{3} C_{2}

MXene@C@SnS hybrids with excellent lithium ion storage performance is very promising anode material for advanced energy storage.
通过在多层

{Ti}_{3} C_{2}

MXene的表面和层间均匀原位生长SnS纳米颗粒，并用碳纳米层封装，合成了这种锂离子电池。具有大比表面积的层状岩层结构为电化学反应提供了丰富的反应活性位点，而大量的纳米孔隙则缩短了电解质的传输路径，促进了锂离子的扩散。这种坚固的结构可减轻快速锂化/去锂化过程中因体积大幅波动而产生的压力。

{Ti}_{3} C_{2}

MXene@C@SnS电极在

0.1 {Ag}^{- 1}

条件下可实现

1473 mA h^{- 1}

的极高可逆容量，在

5 {Ag}^{- 1}

条件下可实现

640 mA h g^{- 1}

的显著速率性能。此外，

{Ti}_{3} C_{2}

MXene@C@SnS 电极在 1 A

g^{- 1}

的条件下循环 350 次以上仍能保持

1050 mA h g^{- 1}

的超高容量，循环稳定性极佳。动力学分析表明，

{Ti}_{3} C_{2}

MXene@C@SnS 电极的锂离子存储主要由表面伪电容行为和扩散控制过程主导。该研究为制备具有多孔层状岩层结构的复合材料提供了新思路，

{Ti}_{3} C_{2}

MXene@C@SnS混合材料具有优异的锂离子存储性能，是非常有前景的先进储能负极材料。

Declaration of competing interest
利益冲突声明

The authors declare no competing financial interest.
作者声明不存在任何经济利益冲突。

CRediT authorship contribution statement
CRediT 作者贡献声明

Hong Tang: Conceptualization, Methodology, Writing - original draft, Investigation, Data curation, Writing - review & editing, Visualization. Ronghui Guo: Conceptualization, Data curation, Funding acquisition, Project administration, Resources, Supervision, Writing review & editing. Mengjin Jiang: Conceptualization, Resources, Investigation. Yue Zhang: Data curation, Validation, Writing - review & editing. Xiaoxu Lai: Writing - review & editing, Visualization, Investigation, Validation. Ce Cui: Writing - review & editing, Visualization, Investigation, Validation. Hongyan Xiao: Investigation, Data curation. Shouxiang Jiang: Investigation, Visualization. Erhui Ren: Investigation, Resources, Visualization. Qin Qin: Investigation, Data curation.
唐红概念化、方法论、写作--原稿、调查、数据整理、写作--审阅与编辑、可视化。郭荣辉概念化、数据整理、资金获取、项目管理、资源、监督、写作审核与编辑。蒋梦瑾：构思、资源、调查。张玥数据整理、验证、写作--审阅和编辑。黎晓旭写作 - 审核与编辑、可视化、调查、验证。崔策写作-审核与编辑、可视化、调查、验证。肖红艳：调查、数据整理。Shouxiang Jiang：调查、可视化。任尔辉：调查、资源、可视化。秦琴：调查、数据整理调查、数据整理

Acknowledgment 鸣谢

This work was financially supported by Joint Fund of the National Natural Science Foundation of China (No. U1833118) and Sichuan Science and Technology Program (2019YFG0244).
本研究得到了国家自然科学基金联合基金（编号：U1833118）和四川省科技计划项目（2019YFG0244）的资助。

Appendix A. Supplementary data
附录 A.补充数据

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.228152.
本文的补充数据可在线查阅：https://doi. org/10.1016/j.jpowsour.2020.228152。

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- Corresponding author. 通讯作者：
E-mail address: ronghuiguo214@126.com (R. Guo).
电子邮件地址：ronghuiguo214@126.com (R. Guo)。

Construction of Ti 3 C 2 Ti 3 C 2 Ti_(3)C_(2)\mathrm{Ti}_{3} \mathrm{C}_{2} MXene@C@SnS with layered rock stratum structure for high-performance lithium storage 构建具有层状岩层结构的 Ti 3 C 2 Ti 3 C 2 Ti_(3)C_(2)\mathrm{Ti}_{3} \mathrm{C}_{2} MXene@C@SnS，实现高性能锂存储

H I G H L I G H T S

A R T I C L E I N F O

Keywords: 关键词：

A B S T R A C T

1. Introduction 1.导言

2. Experimental section 2.实验部分

2.1. Preparation of multilayer Ti 3 C 2 Ti 3 C 2 Ti_(3)C_(2)\mathrm{Ti}_{3} \mathrm{C}_{2} MXene2.1.制备多层 Ti 3 C 2 Ti 3 C 2 Ti_(3)C_(2)\mathrm{Ti}_{3} \mathrm{C}_{2} MXene

2.2. Synthesis of T i 3 C 2 T i 3 C 2 Ti_(3)C_(2)T i_{3} C_{2} Mxene@C@SnS hybrids2.2.合成 T i 3 C 2 T i 3 C 2 Ti_(3)C_(2)T i_{3} C_{2} Mxene@C@SnS 杂化物

2.3. Materials characterization2.3.材料特征

2.4. Electrochemical measurement2.4.电化学测量

3. Results and discussion3.结果和讨论

4. Conclusions 4.结论

Declaration of competing interest利益冲突声明

CRediT authorship contribution statementCRediT 作者贡献声明

Acknowledgment 鸣谢

Appendix A. Supplementary data附录 A.补充数据

References 参考资料

Construction of ${Ti}_{3} C_{2}$ MXene@C@SnS with layered rock stratum structure for high-performance lithium storage
构建具有层状岩层结构的 ${Ti}_{3} C_{2}$ MXene@C@SnS，实现高性能锂存储

2.1. Preparation of multilayer ${Ti}_{3} C_{2}$ MXene
2.1.制备多层 ${Ti}_{3} C_{2}$ MXene

2.2. Synthesis of $T i_{3} C_{2}$ Mxene@C@SnS hybrids
2.2.合成 $T i_{3} C_{2}$ Mxene@C@SnS 杂化物

2.3. Materials characterization
2.3.材料特征

2.4. Electrochemical measurement
2.4.电化学测量

3. Results and discussion
3.结果和讨论

Declaration of competing interest
利益冲突声明

CRediT authorship contribution statement
CRediT 作者贡献声明

Appendix A. Supplementary data
附录 A.补充数据