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This study highlights the importance of the anion in the electrochemical uptake of lithium by metal phosphides. It is shown through a variety of <em>in-situ</em> and <em>ex-situ</em> analytical techniques that the redox active centers in three different systems (MnP<i>₄</i>, FeP<i>₂</i>, and CoP<i>₃</i>) are not necessarily cationic but can rest almost entirely upon the anionic network, thanks to the high degree of covalency of the metal-phosphorus bond and strong P-character of the uppermost filled electronic bands in the phosphides. The electrochemical mechanism responsible for reversible Li uptake depends on the transition metal, whether a lithiated ternary phase exists in the phase diagram with the same M:P stoichiometry as the binary phase, and on the structure of the starting phase. When both binary and lithiated ternary phases of the transition metal exist, as in the case of MnP<i>₄</i> and Li<i>₇</i>MnP<i>₄</i>, a semi-topotactic phase transformation between binary and ternary phases occurs upon lithium uptake and removal. When only the binary phase exists two different behaviours are observed. In the FeP<i>₂</i> system, lithium uptake leads to the formation of an amorphous material in which short-range order persists; removal of lithium reforms some the long-range order bonds. In the case of CoP<i>₃</i>, lithium uptake results in phase decomposition to metallic cobalt plus lithium triphosphide, which becomes the active material for the subsequent cycles.

This study highlights the importance of the anion in the electrochemical uptake of lithium by metal phosphides. It is shown through a variety of <em>in-situ</em> and <em>ex-situ</em> analytical techniques that the redox active centers in three different systems (MnP<i>₄</i>, FeP<i>₂</i>, and CoP<i>₃</i>) are not necessarily cationic but can rest almost entirely upon the anionic network, thanks to the high degree of covalency of the metal-phosphorus bond and strong P-character of the uppermost filled electronic bands in the phosphides. The electrochemical mechanism responsible for reversible Li uptake depends on the transition metal, whether a lithiated ternary phase exists in the phase diagram with the same M:P stoichiometry as the binary phase, and on the structure of the starting phase. When both binary and lithiated ternary phases of the transition metal exist, as in the case of MnP<i>₄</i> and Li<i>₇</i>MnP<i>₄</i>, a semi-topotactic phase transformation between binary and ternary phases occurs upon lithium uptake and removal. When only the binary phase exists two different behaviours are observed. In the FeP<i>₂</i> system, lithium uptake leads to the formation of an amorphous material in which short-range order persists; removal of lithium reforms some the long-range order bonds. In the case of CoP<i>₃</i>, lithium uptake results in phase decomposition to metallic cobalt plus lithium triphosphide, which becomes the active material for the subsequent cycles.

Over the past few years, energy storage devices have received tremendous interest due to the increasing demand for sustainable and renewable energy in modern society. Supercapacitors are considered as one of the most promising energy storage devices because of their high power density and long cycle life. However, low energy density remains as the main shortcoming for supercapacitors. The overall performance of supercapacitors is predominantly determined by the characteristics of the electrodes. Specifically, constructing nanostructured electrode material has been proven as an efficient way to improve the performance by providing large surface area and short ionic and electronic diffusion paths. Another approach to improve the performance of supercapacitors is the rational design of the asymmetric supercapacitor (ASC), which can extend the operation voltage. In this regard, we have focused on the synthesis and utilization of several nanomaterials, in particular, pseudocapacitance materials such as metal oxides and metal phosphides, on both positive and negative electrodes, as well as the ASC design and fabrication. First, three-dimensional TiO2 nanorod arrays have been synthesized on Ti substrate by a facile one-step hydrothermal method and demonstrated as an ideal supercapacitor positive electrode, which exhibited good areal capacitance and excellent cycling stability. Owing to the novel “dissolve and grow” mechanism, this method provides a simple and low-cost technique for flexible supercapacitor application. Second, using cobalt phosphide and iron phosphide as examples, we have demonstrated metal phosphides as high-performance supercapacitor negative electrodes for the first time. Cobalt phosphide nanowire arrays have been synthesized and presented a high capacitance of 571.3 mF/cm2. To improve the cycling stability, gel electrolyte was used to suppress the irreversible electrochemical reaction. The flexible solid-state asymmetric MnO2//CoP supercapacitor exhibited good electrochemical performance, such as a high energy density of 0.69 mWh/cm3 and a high power density of 114.2 mW/cm3. Furthermore, a PEDOT coating has been adapted to further enhance the cycling stability as well as capacitance performance of FeP nanorod arrays. The FeP/PEDOT electrode represents an outstanding capacitance of 790.59 mF/cm2 and a good stability of 82.12% retention after 5000 cycles. In addition, a MnO2//FeP/PEDOT ASC was fabricated with an excellent volumetric capacitance of 4.53 F/cm3 and an energy density of 1.61 mWh/cm3.