【作者】 吴惠明；

【导师】 涂江平；

【作者基本信息】 浙江大学 ， 材料科学与工程， 2007， 博士

【摘要】 锂离子电池具有高电压、比能量高、无记忆效应、无环境污染等特点，已经成为21世纪绿色电池的主要选择。目前商业化使用的锂离子电池正极材料LiCoO₂，由于钴储量有限，价格昂贵，毒性大，作为锂离子电池正极材料成本高和安全性问题，严重阻碍了锂离子电池的进一步发展，限制了它在更广领域的应用。迫切需要研究者开发出成本低，性能优良的锂离子电池正极材料以满足电动汽车等新兴行业的需求。本文主要研究价格低廉的尖晶石LiMn₂O₄系和性能优异的层状LiNiO₂系正极材料。通过采用新型的制备方法和元素掺杂、取代改性等手段来改善这两个系列产物的电化学性能。用XRD、SEM、TEM等表征合成产物的结构特性，结合电化学性能的结果，研究元素掺杂、取代导致的结构变化对材料电化学性能的影响。以醋酸盐为原料，通过喷雾干燥法合成了结晶好，细小、颗粒分布均匀的单相尖晶石LiMn₂O₄粉体。分析了煅烧温度和时间对LiMn₂O₄的结构、形貌和电化学性能的影响。研究表明，喷雾干燥法得到的前驱物在空气中750℃下煅烧24h，制备的LiMn₂O₄晶体生长完整，并具有最好的电化学性能。对比相同热处理条件下固相法制备的LiMn₂O₄，喷雾干燥法所制备的LiMn₂O₄颗粒分布均匀，在室温下具有更好的电化学性能，首次放电容量和高倍率放电能力都有所提高。喷雾干燥法及750℃下煅烧24h制备的LiMn₂O₄在0.1C下首次放电容量达到123mAh g^-1，2C下首次放电容量为107mAh g^-1。采用水热法一步直接制备了尖晶石LiMn₂O₄，省略了在高温下煅烧前驱物的过程。水热法直接制备的尖晶石LiMn₂O₄在0.1C下首次放电容量为121 mAh g^-1，在0.1C下经过40次循环以后，容量维持在111mAh g^-1以上。采用喷雾干燥法合成了掺杂Ni²⁺（0.01≤x≤0.06）的尖晶石LiMn_2-xNi_xO₄，研究了掺杂对尖晶石材料结构和电化学性能的影响。与未掺杂的LiMn₂O₄相比，Ni（2+）掺杂LiMn_2-xNi_xO₄中Mn的平均化合价提高，Mn³⁺数量减少，可以抑制Jahn-Teller效应和减少Mn的溶解损失。电化学测试结果表明，随着Ni含量的增加，尖晶石LiMn_2-xNi_xO₄在常温下的放电容量减小，但循环性能得到改善。Ni²⁺的取代量控制在0.1≤x≤0.5，喷雾干燥法制备了富含Ni²⁺的LiMn_2-xNi_xO₄。XRD结构分析，LiMn_2-xNi_xO₄仍为单一尖晶石结构，其晶格常数与Ni²⁺的取代量x呈线性减小的关系，晶格常数变小，晶胞收缩。同时，随着Ni²⁺的取代量x变化，LiMn_2-xNi_xO₄的电化学性能也将发生明显的变化。研究表明，随着Ni²⁺的取代量x增加，决定4V区平台容量的Mn³⁺离子数量减少，导致4V区平台的充放电容量不断减小。相应的，由于Ni²⁺／Ni⁴⁺氧化还原对数量增加，引起高电压区的充放电容量增加。各电压区间容量的变化，也同Ni离子的取代量呈线性关系。当Ni取代量达到0.5时(产物为LiMn_1.5Ni_0.5O₄)，由于存在少量的O缺失，LiMn_1.5Ni_0.5O₄中仍含有极少量的Mn³⁺离子。其充放电容量除极少量在4V区外，主要集中在高电压区。在常温下的各个充放电区域，尖晶石LiMn_1.5Ni_0.5O₄都具有很好的电化学循环性能。在3.2-4.95V间，其首次放电容量为124mAh g^-1，50次循环以后，容量仍维持在110mAh g^-1以上。对LiMn_1.5Ni_0.5O₄非原位XRD分析结果表明，该材料在此充放电电压范围内，晶体结构不发生改变，只有晶格有限的收缩与扩张。较强的Co-O键，可以加强尖晶石LiMn₂O₄结构的稳定性，从而可能提高电极的循环稳定性。将Co³⁺离子引入到LiMn_1.5Ni_0.5O₄结构中，喷雾干燥法制备了尖晶石LiMn_1.5Ni_0.5-xCo_xO₄（0.1≤x≤0.5），LiMn_1.5-xNi_0.5Co_xO₄（0.1≤x≤0.5）和LiMn_1.5-xCo_2xNi_0.5-xO₄（0.05≤x≤0.25）三个系列化合物，进一步提高5V电极材料LiMn_1.5Ni_0.5O₄的高温循环性能。通过XRD结果分析，三组材料各自都随着Co³⁺离子取代量的增加，晶格常数变小，晶格常数与Co离子呈线性减小的关系。电化学性能测试发现，Mn³⁺／Mn⁴⁺氧化还原对对应充放电过程中的4V平台，Ni²⁺／Ni⁴⁺对应4.6V平台，而Co³⁺／Co⁴⁺则对应5V以上的平台。由此得出，随着Co³⁺离子的取代，化合物中各金属元素的价态会发生变化，引起各个平台之间的变化。由于无法测试5V以上的充放电容量，各系列化合物都随着Co取代量的增加，在3.2-4.95V之间，容量发生衰减。但各系列化合物的高温循环性能得到了提高。LiMn_1.5Ni_0.5-xCo_xO₄系列中，Co³⁺数量增加，则Ni²⁺减少，而Mn³⁺增加，所以造成与Ni²⁺相关的4.6V平台的容量减少，4V区平台的容量增加。当x=0.5时，LiMn_1.5Co_0.5O₄材料在3.2-4.95V的区间，只有4V区的容量，放电容量为66mAh g^-1，高温下经过20次循环后容量几乎没有变化。在LiMn_1.5-xNi_0.5Co_xO₄系列中，Co³⁺取代Mn⁴⁺，随Co³⁺数量x的增加，Mn的化合价不变，有部分的Ni²⁺为Ni³⁺离子，同时也导致该材料在4.6V区的理论容量减小，LiMn_1.5-xNi_0.5Co_xO₄的放电容量分别是119，111，95，80和63mAh g^-1（x=0.1，0.2，0.3，0.4和0.5）。对于LiMn_1.5-xCo_2xNi_0.5-xO₄（0.05≤x≤0.25）系列，Co³⁺同时取代部分的Mn和Ni，发现Mn和Ni的化合价都不随Co³⁺数量的变化而发生改变。尖晶石结构中Mn仍为+4价，Ni仍为+2价，但数量均减少，引起4.6V区平台的容量减少。在LiMn_1.5-xNi_0.5Co_xO₄系列中，LiMn_1.35Co_0.3Ni_0.35O₄在高温下（55℃）首次放电容量为99mAh g^-1，经过20次循环以后容量仍维持在90mAh g^-1以上。本文还采用醋酸盐为原料，通过喷雾干燥法制备了层状LiNiO₂系的正极材料。将喷雾干燥法制备的前驱物在氧气中处理合成了层状LiNi_0.8Co_0.2O₂锂离子电池正极材料。所制备的材料不含别的杂质，结晶良好，颗粒尺寸在200-500 nm之间，晶体结构为层状R-3m。同空气中制备的材料相比，在氧气中制备的LiNi_0.8Co_0.2O₂具有完好的层状结构，（006）／（102）和（108）／（110）分峰明显，表征材料结构中阳离子混排特性的I（003）／I（104）比值高达1.5，说明在氧气中合成的材料具有更好的层状结构，原子的分布得当。在氧气中制备的LiNi_0.8Co_0.2O₂首次放电容量为176mAh g^-1，经过20次循环以后，容量为172mAh g^-1，循环性能良好。掺入+4价Mn离子，降低Ni离子的化合价，通过喷雾干燥法在空气中制备了层状LiNi_1／3Co_1／3Mn_1／3O₂锂离子电池正极材料。前驱物在空气中850℃煅烧24h，所合成的材料具有良好的晶体结构和微观形貌。电化学测试表明，层状LiNi_1／3Co_1／3Mn_1／3O₂在截止电压在4.8V时，首次放电容量为183mAh g^-1。截止电压在4.3V时，首次放电容量为151mAh g^-1，且具有良好的循环性能，经过10次循环后，容量保持率维持在97％以上。更多还原

【Abstract】 Lithium ion batteries have become the primary choice of the green batteries in the 21th century due to its high voltage, high energy density, no memory and non-pollution. At present, the lithium cobaltate, LiCoO₂, is the major cathode active material of commercial lithium ion rechargeable batteries. However, because of its expensive costs and toxicity, it couldn’t meet lithium ion batteries development. Many attempts have been made to develop new cathode materials with lower cost, safety and excellent electrochemical performance. This work aims at developing spinel LiMn₂O₄ and layered LiNiO₂. New synthesized methods, and doping or substituting by metal element are employed to enhance their electrochemical performance. The structure and morphology of these products were investigated by the means of TG/DTA, XRD, SEM and TEM. The electrochemical performances of the compounds have been evaluated by the means of galvanostatic charge-discharge cycling, cyclic voltammetry （CV） and electrochemical impedance spectra （EIS）.Spinel LiMn₂O₄ powder was prepared by a spray-drying method, in which the corresponding acetate salts as the reagents. The product showed a high purity, good crystallinity, small particle size and homogenous size distribution. The effects of synthesis temperature and time on the morphology, structure and electrochemical performances of LiMn₂O₄ had been discussed. It was found that well-crystallized LiMn₂O₄ prepared at 750°C for 24 h in air exhibited the best electrochemical performance. In contrast with LiMn₂O₄ prepared by solid-state reaction with the same sintered process, the LiMn₂O₄ prepared by spray-drying method showed better particle characteristics and better electrochemical performances. Its initial charge capacity and rate capability were improved. The initial capacity was 123 mAh g^-1 at 0.1 C, and at 2 C it also delivered 107 mAh g^-1. Spinel LiMn₂O₄ powder had been successfully synthesized by a hydrothermal method directly, which was no any pretreatment and following treatment in the process. The LiMn₂O₄ delivered reversible capacity of about 121 mAh g^-1 at a current density of 1/10 C. Cycled the cell to 40 cycles, the capacity remained at about 111 mAh g^-1 at 1/2 C.Ni-doped spinel LiMn_2-xNi_xO₄ （0.01≤x≤0.06） was prepared and the dopant effects on structures and electrochemical performances were investigated. Compared with the pure LiMn₂O₄, with increasing the value x of Ni-doped, the Mn average valence in LiMn_2-xNi_xO₄ increased, and the amount of Mn³⁺ decreased, which could restrain the Jahn-Teller effect and reduce the Mn³⁺ dissolution. Through some electrochemical tests, it showed that the capacity of spinel LiMn_2-xNi_xO₄ （0.01≤ x ≤0.06） decreased with the increase of the amounts ofNi-doped, but the cycle capability improved.Abundant Ni-substitated LiMn_2-xNi_xO₄ powders were prepared by spray-drying method, and the x of Ni content was controlled from 0.1 to 0.5. The Ni-substitated LiMn_2-xNi_xO₄ still showed single spinel phase （Fd-3m） without any impurity. The change of lattice parameter was accord to a linearity relation with the Ni-substitated amount. The cell shrunk with the decrease in the lattice parameter. The electrochemical performance would change with the variation of Ni content. When the value x of Ni increased, the amount of Mn³⁺ decreased, which determined the theoretical capacity of 4 V range, and also the amount of the redox pair of Ni²⁺/Ni⁴⁺ increased, which was relevant to the theoretical capacity of 4.6 V plateau. The results showed the factual capacity of LiMn_2-xNi_xO₄ （0.1≤x ≤0.5） had a linearity relation with the Ni-substitated amount. When x was 0.5, the product LiMn_1.5Ni_0.5O₄ still had little Mn³⁺ due to the oxygen absence, and the main capacity happened in 4.6 V plateau. At room temperature, the LiMn_1.5Ni_0.5O₄ exhibited good cycle performance. The initial capacity of LiMn_1.5Ni_0.5O₄ was 124 mAh g^-1, after 50 cycles, it still retained over 110 mAh g^-1 Through ex situ XRD measurement, there were only the shrinkage and enlargement of the lattice parameter, and the structure of LiMn_1.5Ni_0.5O₄ did not change during the charge and discharge process between 3.2-4.95 V.Co³⁺ was doped to stabilize the structure of LiMn_1.5Ni_0.5O₄ due to stronger Co-O bond being than Mn-O and Ni-O. Spinel LiMn_1.5Ni_0.5-xCo_xO₄ （0.1≤x ≤0.5）, LiMn_1.5-xNi_0.5Co_xO₄ （0.1≤ x ≤0.5） and LiMn_1.5-xCo_2xNi_0.5-xO₄ （0.05≤ x ≤0.25） compounds were prepared by spray-drying method. The XRD results showed that lattice parameter of three series compounds were accord to a linearity relation with the Co-doped amount respectively. With increasing the amount of Co, the lattice parameter diminished. After electrochemical tests, it is found that the redox pair of Mn³⁺/Mn⁴⁺ corresponds to the 4 V plateau, Ni²⁺/Ni⁴⁺ corresponds to the 4.6 V plateau and Co³⁺/Co⁴⁺ corresponds to the plateau above 5 V during the charge/discharge process. With increasing the Co³⁺ amount, the valances of the transition metals （Mn, Ni, Co） in the compounds would bring variety, which arose the change of the charge/dischage plateaus. The upper voltage was limited to 4.95 V for fear of possible electrolyte decomposition, especially upon extended cycling, which led to the capacity related to Co³⁺/Co⁴⁺ （which is above 5 V from the CV results） abandon. In the range of 3.2-4.95 V, the discharge capacity would decrease, while all of three series compounds showed good cycle stability at high temperature （55℃）. For the spinel LiMn_1.5Ni_0.5-xCo_xO₄ （0.1≤x ≤0.5）, when the amount x of Co³⁺ increased, the amount of Ni²⁺ decreased, and the amount of Mn³⁺ increased, which led to the theoretical capacity of 4.6 V diminished and the theoretical capacity of 4 V increased. When x =0.5, the capacity of LiMn_1.5Co_0.5O₄ only happened in 4 V range between 3.2-4.95 V. The initial capacity was 66 mAh g^-1, and after 20 cycles thecapacity didn’t any obviously fade. About spinel LiMn_1.5-xNi_0.5Co_xO₄ （0.1≤ x ≤0.5）, when Co³⁺ doped Mn⁴⁺, the valence of Mn didnot change, and partial Ni²⁺ changed to Ni³⁺, which induced the theoretical capacity of 4.6 V decreased. The spinel LiMn_1.5-xNi_0.5Co_xO₄（0.1≤ x ≤0.5） delivered the capacity of 119, 111, 95, 80 and 63 mAh g^-1 （x=0.1, 0.2, 0.3, 0.4 and 0.5）. For the spinel LiMn_1.5-xCo_2xNi_0.5-xO₄ （0.05≤ x ≤0.25）, Co substituted the Mn and Ni site at same time. When the amount of Co³⁺ increased, the valence of both of Mn and Ni retained stability, the amount of Mn⁴⁺ and Ni²⁺ decreased, and the theoretical capacity of 4.6 V decreased. In the series of LiMn_1.5-xCo_2xNi_0.5-xO₄ compounds, LiMn_1.35Co_0.3Ni_0.35O₄ showed the initial capacity value of 99 mAh g^-1 , and after 20 cycles, the capacity obtained over 90 mAh g^-1 at high temperature （55℃）.The layered structure LiNi_0.8Co_0.2O₂ cathode material for lithium-ion batteries was synthesized by sintering the precursor at 750℃ for 24 h in oxygen, which was obtained from the corresponding metal acetate solution via a spray-drying method. Compared with the product prepared in air, the LiNi_0.8Co_0.2O₂ particles obtained in oxygen were fine, narrowly distributed and well crystallized. The product had better layered structure with obvious （108）/ （110） and （006）/ （102） peak splitting and higher ratio of I（003）/I（104） than that prepared in air. As a result, the LiNi_0.8Co_0.2O₂ prepared in oxygen had excellent electrochemical properties. The initial discharge capacity reached 176 mAh g^-1, and its 20th cycle capacity kept to be 172 mAh g^-1 at a current density of 0.1 C.Some Mn⁴⁺ were introduced to layered LiNi_1-xCo_xO₂, the valence of Ni would decreased. The layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode was prepared by spray-drying method. It was found that at at 850℃ for 24 h in air, the obtained layered LiNi_1/3Co_1/3Mn_1/3O₂ showed better crystal structure and better particle characteristics. When the cut-off voltage was controlled at 4.8 V, the initial discharge capacity was 183 mAh g^-1, when the cut-off voltage decreased to 4.3 V, the initial discharge capacity was 151 mAh g^-1, and it showed excellent cycle performance, after 10 cycles, the capacity was over 97% of initial capacity.更多还原