Investigation Of High Capacity Phosphorus-based Bi-active Alloy Materials As Anode For Lithium Ion Batteries

As a new energy storage device,lithium-ion batteries（LIBs）has attracted much attention due to its unique advantages,including high energy density,long cycle life and easy to carry.As an important component,anode shows much influence on the performance of LIBs.Until now,graphite is the most widely used anode material of LIBs,however,its theoretical capacity is limited to 372 mAh g^-1,which is too low to meet the increasing demand of energy.Therefore,the development of high-capacity anode materials has become a research hotspot in the industry of LIBs.As a kind of alloy-type anode material,phosphorus（P）has been widely studied due to its natural abundance and low cost,and it can provide a high theoretical capacity up to 2596 mAh g^-1.However,the poor conductivity of P would result in a poor cycle stability.Therefore,researchers consider introducing another alloy-type material to composite with P and forming dual active P-based alloy to enhance conductivity,thermal stability and chemical stability.As anode materials,they can exhibit high specific capacities and stable cycle performance.In our previous work,we firstly combined germanium（Ge）and phosphorus（P）to form phosphorus germanium bi-active alloy.We found that GeP₅ has a layered structure similar to black phosphorus,which promises a high conductivity close to graphite.And GeP₅ can provide a high theoretical capacity about 2300 mAh g^-1 with a high initial Coulombic efficiency up to 95%.However,GeP₅ experiences a huge volume change nearly to 300%during the charge and discharge processes,which result in a shock damage of electrode structure and poor cycling performance.In addition,the price of Ge is relatively high,and we should further reduce the cost.In the paper,we made an optimization of binder by selecting novel aqueous binder for GeP₅ anode,and the cycle stability of it has been greatly improved.We further introduced Sb metal with lower cost and higher conductivity to replace Ge,forming alloy with P.Sb also shows a layered structure sililiar to graphite,and it experiences a relatively small volume change during discharge/charge process.Based on that,Sb-P alloy exhibits far better electrochemical performance than GeP₅.The specific contents are as follows:（1）As binder can play an important role on the electrochemical performance of alloy-type anodes,we studied in detail the electrochemical performance of GeP₅ with different kinds of binders,and it is found that aqueous binder shows better bonding effect than traditional oil binder PVDF does,and LiPAA is the most suitable one.We further studied the bonding mechanism of LiPAA,and found that the excellent adhesive property of LiPAA binder is mainly attributed to its high fracture strength of molecular structure and the bonding ability with electrode materials.（2）The bi-active alloy is further expanded from Ge-P alloy to Sb-P alloy.We synthesized Sb-P alloy via ball milling method,and studied their lithium ion storage performance for the first time.Compared with the high cost and relatively complex electrochemical reaction process of Ge,Sb 6is cheaper and its lithium storage process is relatively simple.In addition,as the same family element of P,Sb has a layered hexatomic ring structure similar to black phosphorus,which promises high metal conductivity.Moreover,the expansion rate of Sb is relatively small among the alloy-type anode materials,which is far less than that of Si and Ge.As a result,Sb-P alloy shows much better cycle stability than GeP₅.It is proved by theoretical calculation that the P atoms can enter into the lattice of Sb to form displaced solid solution,and the formation energy is relatively low when the ratio of P to Sb is 30:30.We further studied the electrochemical performance of the Sb-P alloy under low temperature test conditions,and found that its excellent properties can be maintained well.Sb-P alloy shows excellent application potential in the new generation of high capacity alloy anode due to its excellent rate performance and cyclic stability.