【作者】 涂健；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料学， 2006， 博士

【摘要】 随着电动汽车（EV）和混合动力汽车（HEV）的发展,尖晶石LiMn2O₄由于成本低、对环境无污染且具有较高的容量,被认为是最有希望取代昂贵、有毒的层状LiCoO₂的锂离子正极材料。然而,尖晶石LiMn₂O₄在循环过程中容量衰减,尤其是在高温下（高于50℃）由于锰的溶解导致容量迅速衰减,严重阻碍了它的商业化进程及其在EV、HEV上的应用。针对这种情况,目前的研究主要趋向于用体相掺杂、表面改性以及优化合成方法来改善尖晶石LiMn₂O₄的循环性能。本文以Li₂CO₃、电解MnO₂为原料,采用高温固相烧结法制备了尖晶石LiMn₂O₄。为了改善尖晶石LiMn₂O₄常温和高温循环性能,采用了金属离子掺杂和熔融浸渍法表面改性的方法,用XRD、SEM、TEM和XAFS等方法表征了离子掺杂和表面改性对尖晶石结构的影响,同时结合了试样的电化学性能,研究了这两种方法所造成的结构变化对尖晶石LiMn₂O₄电化学性能的影响。 本文首先通过高温固相法合成了结晶良好Li/Ni协同掺杂具有尖晶石结构的LiMn_2-2xLi_xNi_xO₄（x=0.03,0.05,0.075,0.1）试样,其晶格常数c随着掺杂量x的增加呈线性减小。通过XRD、FTIR和XAS分析表明,Li/Ni协同掺杂提高了Mn的价态及Mn-O的键能,减小了尖晶石结构中MnO₆八面体的畸变,提高了其结构的稳定性。Ex SituXRD研究表明,与纯尖晶石LiMn₂O₄相比,Li/Ni协同掺杂不但能够有效地抑制高电位平台位置的两相共存现象,而且减小了试样在脱嵌锂 离子过程中的晶格和体积变化。因此Li/Ni协同掺杂的LiMn_2-2xLi_xNi_xO₄试样在室温和高温下的循环性能都得到了有效地改善,且随着掺杂量的增加,循环更加的稳定。 采用稀土元素La对尖晶石LiMn₂O₄进行了掺杂,Rietveld分析显示最多只有1%左右的La能够掺杂进入尖晶石的晶格并占据了16d的位置,而少量的Mn占据了8α的位置。当掺杂量超过1%以后,La与试样中的Mn结合形成LaMnO₃杂质。少量的La掺杂能够在循环过程中有效地稳定尖晶石结构,防止充放电过程中的失氧及晶粒的细化,因而保证了电极具有良好的电接触,抑制了电池阻抗的增加。与LiMn₂O₄相比,LiLa_0.01Mn_1.99O₄电极在室温下循环了近300次后,容量还保持了首次容量的90.5%,平均容量在110mAhg^-1以上,显示了良好的循环性能。 然而,体相掺杂所带来的循环性能的改善是以牺牲可逆放电容量为代价的,如溶胶-凝胶法等表面改性方法虽有效且对可逆容量影响小但在工业化生产上并不可行。为此,我们希望能够找到一个简便而又实用的表面改性方法来更好的改善尖晶石LiMn₂O₄尤其在高温下的循环性能。本文通过硝酸盐熔融浸渍的方法,成功的对尖晶石LiMn₂O₄进行了表面包覆改性。对比LiMn₂O₄试样,表面包覆改性LiMn₂O₄试样在室温和高温电化学性能得到了极大的改善。LiMn₂O₄/ZnO试样在55℃循环具有最低的容量衰减率(每更多还原

【Abstract】 Lithium ion batteries are undergoing a rapid and innovative development due to increasing market demand for EVs and HEVs. The layered oxides, LiCoO₂, LiNiO₂ and spinel LiMn₂O₄ have been widely studied as cathode materials for lithium ion batteries. Among them, spinel LiMn₂O₄ is considered to be one of the most promising candidates due to its low cost, nontoxicity and easy preparation. However, spinel LiMn₂O₄ suffered from severe capacity fading during cycling, especially at elevated temperatures （higher than 50°C）, which hindered its commercialization. Doping or suface modification was adopted to overcome this problem in previous studies. In this dissertation, spinel LiMn₂O₄ with Fd3m group was synthesized by high temperature solid state reaction, and the effect of doping or surface modification on the structure and electrochemical performances at both room temperature （RT） and 55℃ have been investigated. The improved performances of these samples at room and elevated temperatures ascribed to structural and chemical changes resulting from doping or surface modification.Sample LiMn_2-2xLi_xNi_xO₄（x=0.03, 0.05, 0.075, 0.1） were synthesized with well-defined octahedral configuration and lattice parameters of them decreased linearly with the increase of x. XRD, FTIR and XAS ananlyses show that Li/Ni co-doping stabilizes the structure of spinel LiMn_2-2xLi_xNi_xO₄ by increasing the valence of Mn and decreasing the distortion of Mn³⁺O₆. Ex Situ XRD shows that Li/Ni co-doping can suppress the two-phase co-existence in higher voltage plateau and reduce the lattice and volume change during electrochemical cycling. Compared with pure LiMn₂O₄, Samples LiMn_2-2xLi_xNi_xO₄（x=0.03, 0.05, 0.075, 0.1） show better performance at both RT and 55℃. The cyclability data become better with the increase of x.1%, 2.5% and 4% （atom ratio） La-modified spinel LiMn₂O₄ were prepared and Rietveld refinement shows that ca. 1% can be doped into \6d sites of the lattice. After the doped amount exceeded 1%, the impurity LaMnO₃ was formed. La doping, which suppresses the oxygen losses and loss of crystallinity during the charge-discharge process, inhibits the structural destruction as well as the increase of the electrode impedance during prolonged cycling, and thus good performances can be achieved. Compared with spinel LiMn₂O₄, sample LiLao.01Mn1.99O4 show excellent capacity retention with only 9.5% capacity loss after 300 cycles at RT.Cationic substitution improved the room-temperature performance significantly, but the improvement was achieved at the expense of the reversible capacity, oxides coating such as ZnO, MgO etc are effective in improving performances at 55℃, but the coating procedures adopted in above studies were based on the sol-gel or micro-emulsion methods, which are beset with problems such as the high cost of alkoxide oxide precursors and questionable industry-scale production. In this dissertation, the surface of as-prepared LiMn₂O₄ was modified successfully by a melting impregnation method, which can improve the cycleability greatly at both RT and 55℃. Among all samples, Sample LiMn₂O₄/ZnO exhibits the lowest capacity fading rate （0.064% per cycle over 100 cycles） at 55°C, and samples LiMn₂O₄/LiAlO₂ and LiMn₂O₄/Al₂O₃ Show the best capacity retention ability （87.5% after 300 cycles and 96.3% after 100 cycles, respectively）. Comparing with LiMn₂O₄ and LiAl_0.04Mn_1.96O₄, TEM and XAS analyses demonstrate that not only nano-Al₂O ₃ is coating on the surface of LiMn₂ O₄/Al₂O₃, but also a substitutional shell has formed. The improvement of surface modified samples by a melting impregnation method is due to suppression of the surface Jahn-Teller distortion and a slow-down of manganese dissolution by the existence of substitutional shell and oxide coating on the surface, hence keeping good structural stability and good electric contact.At last, as a combination of doping and surface modification, sample LiAl_0.04Mn_1.96O₄ was modified by LiCoO₂ or ZnO with a melting impregnation method. XRD and TEM show that Li/Co atoms have diffused into the bulk after surface modification with 3%, 5% and 8% LiCoO₂. The chemical diffusion coefficiences of surface modified samples are about 3.2×10¹⁹¹.3×l0^-11 cm² s^-1, which is one order magnitude higher than sample LiAl_0.04Mn_1.96O₄, and thus the surface modified sample show better rate characteristics. 5% ZnO-modified sample, LA/ZnO, shows a capacity of 140 mA h g^-1 between 2.6 and 4.35 V without remarkable capacity loss over 50 cycles despite the Jahn-Teller distortion at 3 V. High temperature aging experiment shows that the ZnO on the surface can suppress the dissolution of Mn and keep the structure intact. Sample LA/ZnO exhibits better performance at both room temperature and ℃ with a capacity fading of 0.036% at RT and 0.041% at 55℃.更多还原