Design And Synthesis Of Layered MoS₂ And MoSe₂ Based Composites As Advanced Anodes For Lithium Or Sodium Ion Batteries

The application of the transition metal dichalcogenides(TMDs)-based composites for lithium/sodium storage is limited by their unsatisfactory rate performance and cycling durability.In our research,by using methods including nano structuring,surface modification and carbon hybridization,we rationally design and controllably fabricate multilayer structures for representative TMDs including MoS2 and MoSe2 to improve their lithium/sodium storage performance.We further investigated their underlying lithium/sodium storage mechanism by combing XPS and HRTEM analysis.The theoretical insight gained in this work could have general impact on other TMDs used in energy storage.(1)The free-standing and free-binder porous carbon coated MoS2/N-doped reduced graphene oxide composite(C-MoS2/N-doped rGO)aerogel is constructed through two-step hydrothermal method using NH3 as nitrogen source and PEG-20000 as carbon source.In such structure,double modifications for MoS2-based electrode are realized by carbon coating and rGO skeleton,thus the C-MoS2/N-doped rGO electrode display enhanced electrochemical performances for lithium storage,maintaining 900 mAh/g after 500 cycles at a current density of 200 mA/g.Superior high-rate capability is achieved for the C-MoS2/N-G with a reversible capacity of 500 mAh/g at 4 A/g.Furthermore,the lithium storage mechanism of the MoS2-based integrated electrode is investigated by ex-situ XPS and HRTEM technologies in detail.(2)The unique 3D hierarchical MoS2-PPY-rGO films supported by Cu foil were fabricated by solution-based method.Double modifications of MoS2 nanosheets are realized in this film by introducing PPY layer with the thickness of 5-10 nm for prohibiting the solution of polysulfides and rGO as highly conductive carbon skeleton.As a free-binder electrode for lithium-ion batteries,the MoS2-PPY-rGO composite maintains 1070 mAh/g after 400 cycles at a current density of 200 mA/g,and also exhibits superior rate capacity of 600 mAh/g at 2 A/g._Additionally,the lithium storage performances of the these electrodes with different mass loading are performed and it is found that MoS2-PPY-rGO film with mass loading of 2 mg/cm2 exhibits superior cycling stability,indicating that the thickness of film electrode is vital to electrochemical performance in the energy storage.(3)Inspired by the biological structures in nature,hydrothermal MoS2 nanosheets and polymerized N-doped carbon are rationally assembled on the loofah sponge-derived carbon microtubes(LSDCM)to form a 3D ternary sandwiched LSDCM/MoS2/N-C composite.As anodes for lithium or sodium ion batteries,the LSDCM/MoS2/N-C ternary composites display significantly enhanced electrochemical performance due to the smart design and unique porous ternary structure,including highly reversible capacity,superior capability and excellent capacity retention.For lithium storage,the LSDCM/MoS2/N-C ternary electrode delivers 1058 mAh/g after 500 cycles at a current density of 200 mA/g,and also exhibits superior rate capacity of 600 mAh/g at a high current density of 4 A/g,much better than LSDCM/MoS2,MoS2 and LSDCM electrodes.Additionally,a long-term high-rate performance is obtained(571 mAh/g after 320 cycles at 4 A/g).For sodium storage,the LSDCM/MoS2/N-C ternary electrode maintains 534 mAh/g after 100 cycles at 200 mA/g,and also exhibits superior rate capacity of 245 mAh/g at 4 A/g.(4)Scrupulous design and synthesis of high performance electrode materials are vital for developing sodium ion batteries.Thus,3D porous net-work structured carbon coated MoSe2/rGO aerogel(C-MoSe2/rGO)is constructed through a facile one-step hydrothermal strategy using glucose as carbon source and graphene oxide as carbon skeleton.The MoSe2 nanosheets wrapped by thin carbon layer are strongly incorporated into 3D porous reduced graphene oxide aerogel architecture,thus achieving double modification for MoSe2 electrodes.As a anode for sodium ion batteries by directly cut and compressed into a circular pellet,the C-MoSe2/rGO electrode delivers 445 mAh/g after 500 cycles at a current density of 200 mA/g.Superior high-rate capability is achieved with a reversible capacity of 225 mAh/g at 4 A/g.Additionally,the sodium storage mechanism is investigated well using ex-situ XPS and HRTEM methods in detail.(5)Three-layer sandwiched VG/MoSe2/N-C core/shell arrays are fabricated by hydrothermal MoSe2 nanosheets and polymerized N-doped carbon via combined hydrothermal and polymerization approaches.Wrinkled MoSe2 nanosheets are vertically growth on vertical graphene(VG)and then wrapped by N-doped carbon(N-C)shell forming sandwiched core/shell arrays.The sandwiched active MoSe2 enjoys multiple benefts including omnibearing conductive networks from interconnected VG arrays and N-C layer,structural protection,and suppression of active materials loss from N-C shell,as well as large porosity.As a preliminary test for sodium ion batteries,the VG/MoSe2/N-C ternary electrode maintains 545 mAh/g after 400 cycles at a current density of 200 mA/g,and also exhibits superior rate capacity of 295 mAh/g at 2 A/g.Especially,the long-term high-rate performance is achieved,maintaining 398 and 298 mAh/g after 1000 cycles at 1 and 2.0 A/g,respectively.