Li Qiyuan, Wen Zheng, Fan Changling, Zeng Taotao, Han Shaochang
College of Materials Science and Engineering, Hunan University Changsha 410082 China
Hunan Province Key Laboratory for Advanced Carbon Materials and Applied Technology, Hunan University Changsha Hunan 410082 China.
RSC Adv. 2018 Feb 13;8(13):7044-7054. doi: 10.1039/c8ra00370j. eCollection 2018 Feb 9.
A new cathode LiVPOF/C with a high working voltage of around 4.2 V was synthesized by a novel one-step method. The color of the solution turns green, which implies that VO is successfully reduced to V. The reaction thermodynamics indicates that LiVPOF/C is formed when the sintering temperature is higher than 650 °C, while the accompanying impurity phase LiV(PO)/C is also generated. The reaction kinetics proves that the reaction is third order and the activated energy is 208.9 kJ mol. X-ray photoelectron spectra imply that the components of LiVPOF/C prepared at 800 °C (LVPF800) are in their appropriate valence. LVPF800 is composed of micron secondary particles aggregating from nano subglobose. The structural transformation shows that the V : P : F ratio in LVPF800 is close to 1 : 1 : 1. The reason behind generation of impurity LiV(PO) at a high temperature of 850 °C is demonstrated directly, which is mainly due to the volatilization of VF. The electrochemical performances of the cathode are related to the crystallite content of LiVPOF/C and LiV(PO)/C. The specific capacities at 0.2 and 5C of LVPF800 are as high as 139.3 and 116.5 mA h g. Electrochemical analysis reveals that LVPF800 possesses an excellent reversibility in the extraction and insertion process and minimum charge transfer resistance.
采用一种新型的一步法合成了一种新型的工作电压约为4.2 V的阴极LiVPOF/C。溶液颜色变为绿色，这表明VO成功还原为V。反应热力学表明，当烧结温度高于650℃时形成LiVPOF/C，同时还生成了伴随的杂质相LiV(PO)/C。反应动力学证明该反应为三级反应，活化能为208.9 kJ mol。X射线光电子能谱表明，在800℃制备的LiVPOF/C（LVPF800）的组分处于合适的价态。LVPF800由纳米亚球形团聚而成的微米级二次颗粒组成。结构转变表明，LVPF800中V : P : F的比例接近1 : 1 : 1。直接证明了在850℃高温下产生杂质LiV(PO)的原因，主要是由于VF的挥发。阴极的电化学性能与LiVPOF/C和LiV(PO)/C的微晶含量有关。LVPF800在0.2C和5C时的比容量分别高达139.3和116.5 mA h g。电化学分析表明，LVPF800在脱嵌过程中具有优异的可逆性和最小的电荷转移电阻。
