煅烧时间对正极材料LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2)浓度梯度结构及电化学性能的影响

Effect of calcination time on structure and characterization of a full concentration-gradient LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2) cathode material

将铝溶液按3种不同的流速连续注入,联合共沉淀结晶控制法合成了具有核壳结构的Ni_(0.80)Co_(0.15)Al_(0.05)(OH)_(2)前驱体,即内核为均相组成的Ni_(0.88)Co_(0.12)(OH)_(2),外壳为Ni、Co和Al含量呈连续变化的Ni_(0.72)Co_(0.18)Al_(0.10)(OH)_(2)。将该前驱体与LiOH·H_(2)O混合均匀,在700℃的氧气气流下进行煅烧,通过控制煅烧时间使材料中Ni、Co和Al扩散形成含量呈连续梯度变化的且具有类球形形貌的LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2)正极材料。梯度结构中Ni、Co和Al的分布情况直接影响材料的电化学性能和结构稳定性。当煅烧时间为12 h时,从颗粒的核心到表面,Ni含量(原子分数)由0.855下降至0.732,Al含量由0.003增加至0.115,Co含量维持在0.142~0.163之间,而表面组成为Ni_(0.732)Co_(0.153)Al_(0.115)O_(2)。此时材料具有良好完整的层状结构且Li^(+)/Ni^(2+)混排程度最低,在0.2C下的放电比容量为201.3 m Ah·g^(-1),略低于均相LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2)的放电比容量(205.8 mAh·g^(-1));以0.2C充电、1C放电循环200周后,其容量保留率为71.6%,优于均相组成的材料(54.6%)。这是因为镍含量低而铝、钴含量高(相对于均相材料)的外层可以有效抑制充放电循环过程中引起颗粒体积的各向异性变化,减少电极极化,从而减缓极片表面裂纹的生成和扩展,降低电池的电荷传递阻抗,从而提高循环稳定性和结构稳定性。该材料合成过程中只有在铝溶液流速切换时使p H值发生小幅度的波动,但很快恢复到稳定,可以保证前驱体具有良好的结晶性,使核壳结构易设计、控制;而后通过调控煅烧时间合成LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2),有利于进一步地精准设计材料的浓度梯度结构。

Core-shell structure precursor LiNi_(0.80)Co_(0.15)Al_(0.05)(OH)_(2) was synthesized via co-precipitation by adjusting the Al solution with three flow rates(Each flow rate worked for three hours).The core of the precursor was a uniform structure with a composition of Ni_(0.88)Co_(0.12)(OH)_(2);and the shell of the precursor was a concentration-gradient structure with a composition of Ni_(0.72)Co_(0.18)Al_(0.10)(OH)_(2),which Ni content decreased gradually while Al content increased steadily from the surface of the core to the particle surface.A mixture of this core-shell structure precursor and LiOH·H_(2)O was sintered at 700℃for different sintering times in the O_(2) atmosphere to obtain a full concentration gradient and spherical LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2).The diffusion of Ni,Co,and Al under different calcination times led to a concentration gradient variation of LiNi_(0.80)Co_(0.15)Al_(0.05)O_(2),which displayed different electrochemical performance.When the sintering time was 12 h,the obtained material had a well-designed concentration-gradient structure:from the core to particle surface,the content(atomic fraction)of Ni decreased from 0.855 to 0.732,while the content of Al content increased steadily from 0.003 to 0.115.And the content of Co first increased from 0.142 to 0.163 and then decreased to 0.153.This cathode material had a lower degree of cation mixing and well‑developed layered characteristics.It had a discharge capacity of 201.3 mAh·g^(-1) at 0.2C,which was just under 205.8 mAh·g^(-1) of the homogeneous one;and it showed excellent capacity retention of 71.6%after 200 cycles,which was much higher than that of the homogeneous material(54.6%).It is attributed to a good Ni‑deficient and Al/Co‑rich out‑layer,which can reduce anisotropic volume variations and electrode polarization during cycling.As a result,it could reduce the charge‑transfer resistance of the electrode,and prevent the formation and extension of micro‑cracks on the electrode's surface.This cathode material is easy for industrial application because the pH value of the co‑precipitation reaction is quite stable and its concentration gradient structure is controllable by adjusting the flow rates of the Al solution and the calcination time.Furthermore,the pH value of the co‑precipitation reaction shifted only when the injection rate of the Al solution was adjusted.After that,the pH value quickly returned to keep constant,and the precursor had high crystallinity and good consistency of the material.