| Nano-powders of LiMn2O4 with spinel structure were prepared using nitrate and acetate as raw materials adopting sol-gel method. Nano-powders of LiMn2-xMxO4-yFy with spinel structure were also prepared using Cr, Ni, F as doping elements. DTA-TG, XRD, SEM, BET, ICP-AES were adopted to analyze and characterize the thermal behavior of precursor and physics properties of the prepared products. The judgement method of confused type and degree of atom occupied locations in LiMn2O4 were investigated. The effects of preparation process on occupied location of atom in LiMn2O4 and electrochemistry properties were investigated. Meanwhile, the mechanisms of capacity decay were also investigated.The results show that the decomposition speed of precursor is rapid using nitrate as raw materials. The purity of LiMn2O4 prepared is relatively low and the influence of sintering temperature on purity of LiMn2O4 is great, so the qualities of LiMn2O4 can not easily be controlled. However, the decomposition speed of precursor is slow using acetic acid salt as raw materials. The purity of LiMn2O4 prepared is relatively high and there is no obvious influence of sintering temperature on purity of LiMn2O4, so the qualities of LiMn2O4 can be easily controlled. The prepared powders are global and their average sizes are about 40nm.With the increase of sintering temperature, the lattice constant and particle size of LiMn2O4 increase, while the purities of LiMn2O4 tend to reduce. Pretreatment at 300℃can obviously enhance the purity of LiMn2O4 and fine the particle sizes.When the peak intensity ratio of I111/I311 in spinel LiMn2O4 reduces, the confused degree of atom occupied locations increases. The type of confused degree depends on the variation tendencies of I311/I400 and I111/I311 value. If their variation tendency is the same, it is the 16c type and if their variation tendency is opposite, it belongs to anti-spinel type. Under the used experimental condition, when sintering temperature is low, it is apt to produce location confusion in anti-spinel type. With the increase of sintering temperature, confused degree in anti- spinel type deduces, while the confused degree in 16c type increases to some extent. Excessive lithium ion occupies 16d position and forms rich lithium spinel Li4Mn5O12 with the structural formula [Li]8a[Mn1.67Li0.33]16dO4.When the occupied location confusion with anti-spinel type takes place in spinel LiMn2O4, initial discharge capacity and cycling properties reduce, While occupied location confusion with 16c type does not influence the charge and discharge properties. LiMn2O4 structure takes place transformation from cubic to tetragonal d (c/a =1.642, a =0.5760nm, c =0.9459nm) during charge and discharge, which results in the capacity decay of LiMn2O4. Rich lithium Li4Mn5O12 with spinel structure has higher intial discharge capacity and cycling properties. The discharge capacity in the region of 3V is relatively large, while it is very small in the region of 4V.The spinel structure of LiMn2O4 can be stabilized after doping a small amount of Cr3+ or Ni2+ and Jahn-Teller effect can be suppressed, which significantly improves its cycling properties. But it will lose some initial capacity after doping some Cr3+ or Ni2+. The more of doped amount and the lower of doped element chemical valence are, the greater losses of the initial capacity are. The quantities of Mn3+ in LiMn2O4 can be increased by doping F- , which improves the initial discharge capacity. However, Jahn-Teller effect can be easily produced and cycling properties become bad because of reducing the average chemical valence of Mn. The capacity retain rate of LiMn2O4-xFx(x =0.3, 0.4, 0.5, 0.6, 0.7) is lower than that of LiMn2O4 and with the increase of F quantity, the capacity retain rate of reduces. But the initial discharge capacity increase and in x =0.5 it reaches the maximum (124.8mAh/g). The initial discharge capacity and cycling properties can be improved by adopting Cr3+ and F- together in LiMn2O4. |
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