Electrochemical Characteristics of Ti–P Composites Prepared by Mechanochemical Synthesis

Abstract

Titanium phosphide composites with various Ti/P molar ratios ͑1:1, 1:2, and 1:4͒ were synthesized by a mechanochemical method and their potential use as an alternative anode material for Li secondary batteries was investigated. The titanium phosphide composites with Ti/P molar ratios of 1:1 and 1:2 did not show any electrochemical reactivity with Li, while both the TiP 2 and P in the composite with a Ti/P molar ratio of 1:4 reacted with Li, yielding a reversible capacity of 1422 mAh/g. Ex situ X-ray diffraction analyses and X-ray absorption spectroscopy showed that the TiP 2 -P composite transformed into cubic Li 10.5 TiP 4 phase without decomposing into Ti and Li 3 P during the initial Li insertion. Subsequent cycling showed a highly reversible insertion/ extraction process of Li to/from Li 10.5 TiP 4 phase, accompanied by the loss/recovery of the long-range cubic order. The cubic Li 10.5 TiP 4 phase showed excellent cycling performance with a large capacity of 625 mAh/g for 80 cycles when the amount of lithium reacted was suitably controlled. and the Li x MP 4 ͑M = Ti, V, and Mn͒ system 15-22 due to their ability to react reversibly with a large amount of Li. Their electrochemical reaction mechanisms with Li have been proposed, and these mechanisms have been found to vary according to the nature of the metal in the metal phosphides and their composition and structure. In the MnP 4 system, 1 a quasi-topotactic Li intercalation process was observed. The P-P bonds in the MnP 4 were cleaved upon reduction to form crystalline Li 7 MnP 4 and reformed to generate MnP 4 upon oxidation. For CoP 3 2,3 and NiP 2 , 4 phase separation was observed when they were discharged completely, yielding nanophase metal embedded in a matrix of Li 3 P. Li can be reversibly extracted from Li 3 P to leave either LiP or P without the participation of the metals. The reaction of FeP 2 5 or CuP 2 7 with Li displays different behavior, forming a metastable ternary Li-Fe-P phase or a new Li 2 CuP phase, respectively. Compared with the binary metal phosphides, stable ternary phase systems such as Li 2 CuP 14 and Li x MP 4 ͑M = Ti, V, and Mn͒ 15-22 were found to show a reversible ordered-disordered phase transformation accompanied by structural rearrangements during repeated Li insertion/extraction. These materials exhibit improved capacity retention, due to the structural stability of the M-P x host matrix. Titanium phosphides are known to have a stable ternary Li-Ti-P phase and can be expected to insert and extract Li reversibly in the titanium phosphide host matrix during cycling. In this work, Ti-P composites were prepared by mechanochemical synthesis with various molar ratios of Ti and P, and the electrochemical behaviors and structural variations of the Ti-P composites were investigated during cycling, in order to assess their potential use as an anode material for lithium secondary batteries. ExperimentalTi-P composites were synthesized as follows. Ti metal ͑99.9%, 250 mesh, High Purity Chemicals͒ and red P ͑99+%, Aldrich͒ were mixed wel...

Beyond Intercalation‐Based Li‐Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

“…[232][233][234][235] As a matter of fact, the attractive redox activity of phosphorus has led to efforts to investigate the performance of a Li-P battery. [ 236 ] While no conversion has been observed for phosphides containing early transition metals such as Ti or V, [237][238][239] …”

Reversible and High‐Capacity Nanostructured Electrode Materials for Li‐Ion Batteries

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