Niobium oxyphosphate nanosheet assembled two-dimensional anode material for enhanced lithium storage

The development of high-capacity and high-rate anodes has become an attractive endeavor for achieving high energy and power densities in lithium-ion batteries(LIBs).Herein,a new-type anode material of reduced graphene oxide(rGO) supported niobium oxyphosphate(NbOPO_4) nanosheet assembled twodimensional composite material(NbOPO_4/rGO) is firstly fabricated and presented as a promising highperformance LIB anode material.In-depth electrochemical analyses and in/ex situ characterizations reveal that the intercalation-conversion reaction takes place during the first discharge process,followed by the reversible redox process between amorphous NbPO_4 and Nb which contributes to the reversible capacity in the subsequent cycles.Meanwhile,the lithiation-generated Li3 PO_4,behaving as a good lithium ion conductor,facilitates ion transport.The rGO support further regulates the structural and electron/ion transfer properties of NbOPO_4/rGO composite compared to neat NbOPO_4, resulting in greatly enhanced electrochemical performances.As a result,NbOPO_4/rGO as a new-type LIB anode material achieves a high capacity of 502.5 mAh g^(-1) after 800 cycles and outstanding rate capability of 308.4 mAh g^(-1) at 8 A g^(-1).This work paves the way for the deep understanding and exploration of phosphate-ba sed high-efficiency anode materials for LIBs.

A chemo-mechanical model for fully-coupled lithiation reaction and stress generation in viscoplastic lithiated silicon

Development of stresses in silicon(Si) anodes of lithium-ion batteries is strongly affected by its mechanical properties. Recent experiments reveal that the mechanical behavior of lithiated silicon is viscoplastic, thereby indicating that lithiation-induced mechanical stresses are dependent on the lithiation reaction rate. Experimental evidence also accumulates that the rate of lithiation reaction is conversely affected by the magnitude of mechanical stresses. These experimental observations demonstrate that lithiation reaction and stress generation in silicon anodes are fully coupled. In this work, we formulate a chemo-mechanical model considering the two-way coupling between lithiation reaction and viscoplastic deformation in silicon nanoparticle anodes.Based on the model, the position of the lithiation interface, the interface velocity, and the lithiation-induced stresses can be solved simultaneously via numerical methods. The predicted interface velocity is in line with experimental measurements reported in the literature. We demonstrate that the lithiation-induced stress field depends on the lithiation reaction through two parameters:the migration velocity and the position of the lithiation interface. We identify a stress-mitigation mechanism in viscoplastic silicon anodes: the stress-regulated lithiation reaction at the interface serves as a "brake" to reduce the interface velocity and mitigate the lithiation-induced stresses, protecting the Si nanoparticle anode from being subjected to excessive mechanical stresses.