Synthesis And Modification Of Cuprous Phosphide For Lithium-ion Battery Anodes

Nowadays, lithium-ion batteties（LIBs） are widely applied in the fields of consumer electronics and power batteries, in consideration of their numerous advantages: high efficiency, high energy density, and so on. As the requirements of LIBs are rising rapidly, cuprous phosphide（Cu₃P） can be a potential anode material for LIBs with its comparable gravimetric capacity（363 mAh/g）, and four times higher volumetric capacity（4732 Ah/L） than graphite. Meanwhile, it has other advantages such as suitable discharge potential（0⁰.5 V v.s. Li+/Li）, abundant reserves of raw materials and low cost. However, Cu₃P has some problems like poor cycling stability and rapidly fading capacity at different current densities. Based on the awareness of the Li-ion intercalation/deintercalation mechanism and the latest research of Cu₃P, a series of studies including synthesis methods, influence of elements ratios and modification are carried out in this paper.Firstly, melting, redox reaction and mechanical alloying are used to synthesise different Cu₃P samples respectively. After simple researches about products’ electrochemical behaviors, their influence on crystalline structure, grain size and electrochemical performance is studied. After comparing the test results, mechanical alloying is a better choice as it can produce samples with outstanding performance convenientely.Then mechanical alloying Cu₃P is used to study that how potential range influences material’s cycling stability and rate capability. Different potential windows（0.5² V and 0.02² V） are chosed. When the cut-off voltage of discharge is 0.02 V, Cu₃P will undergo a more complete redox reaction. The large volume change between the reactant and resultant, as well as a large number of aggregated Cu（“dead Cu”） is produced during the reaction, lead to poor electrochemical performance. Otherwise the 0.5 V cut-off voltage can show higher column efficiency, better cycling stability and rate capability. The differences of electrochemical performance at 360mA/g（1.5 C） current density are most obvious for the reversible capacity of 0.5 V is almost double than the 0.02 V.Although the synthesis method and potential window are optimized, the cycling stability and rate capability of Cu₃P is still poor. So Fe addition is employed as a simple strategy to modulate the composition and phase constitute of Cu₃P nanopowders synthesized by wet mechanical alloying and thereby to tune the electrochemical performance of the anode. The addition of Fe results in a composite consititue containing Cu₃P as the major phase and some other minor phases including Cu, α-Fe and FeP, which are combinationally determined by lots of tests. Electrochemical tests reveal that both the cycling stability and the rate capability of the electrodes are improved by Fe-addition. The Cu₃P electrode with 10 % Fe-addition shows the best cell performance, with the capacity after 50 cycles at 180mA/g（0.75 C） current desity and the capacity at 360mA/g（1.5C） current desity both being remarkably improved by over 100 %. The improvement of the electrochemical performance is engendered by the synergetic effects of the microstructure change of the powders and the presence of Fe-related minor phases, leading to increased conductivity as well as enhanced electrochemical reversibility of the electrode.Finally, this paper discusses about the influence of diffferent element ratios caused by mechanical alloying on electrochemical behavior. Different mechanisms are also analysized. Experiments point out that both excessive Cu and excessive P materials will have negative effects, only the stoichiometric Cu₃P can show better electrochemical performance. According to electrochemical performance, different excessive elements have different contributions on the worse performance of Cu₃P.