art_10.1007_s11581-011-0620-9

锂电池

ORIGINAL PAPER

Gai Yang &Changyin Jiang &Xiangming He &Jierong Ying &Feipeng Cai

Received:24January 2011/Revised:8August 2011/Accepted:28August 2011/Published online:14September 2011#Springer-Verlag 2011

Abstract The preparation of vanadium-modified olivine LiFePO 4was attempted using vanadium-modified FePO 4precursor which was synthesized by controlled crystalliza-tion.The structure and electrochemical behavior of V-LiFePO 4with different vanadium contents were investigat-ed.The electrochemical behavior of V-LiFePO 4materials at high rate and low temperature was compared with that of the LiFePO 4material.Incorporation of vanadium improved the electrochemical performance of LiFePO 4.The investi-gation showed that the 3%V-modified LiFePO 4presented the best electrochemical performance.

Keywords Lithium-ion batteries .LiFePO 4.Vanadium modification .Controlled crystallization

Introduction

Olivine-structured LiFePO 4as proposed by Padhi et al.[1]is recognized as a promising positive electrode material for lithium-ion batteries.It has advantages such as low cost,excellent thermal stability,satisfactory safety and low toxicity,etc.However,pristine LiFePO 4has the disadvan-tage of poor rate performance due to its low electronic conductivity (~10?9S cm ?1).Considerable efforts have

been made to increase its electronic conductivity by the synthesis of LiFePO 4/C composite [2,3]or super-valence ion doping [4].Through these approaches,the electronic conductivity of LiFePO 4was increased to as high as 10?2S cm ?1,even though the increased electronic conductivity did not result in the improvement of the rate performance of LiFePO 4as expected.The main reason may be because the insertion/extraction of Li +was controlled by the Li +chemical diffusion.Accordingly,the ion conductivity of Li +may play a more important role in the rate capability of LiFePO 4.

Minimizing the particle size of LiFePO 4could shorten the path length for Li +transport and enhance the insertion/extraction of Li +reversibly.However,the lithium-ion diffusion rate in LiFePO 4material was not improved by this way.Wang et al.[5]reported that the rate capability and cyclic stability of LiFePO 4were greatly enhanced by bivalent cation (Ni,Co or Mg)doping at Fe site.Under a high rate of 10C at room temperature,the specific capacity of LiFe 0.9Co 0.1PO 4was maintained at 90.4mAh g ?1.Fe site doping increased the ionic mobility and diffusion coefficient probably by weakening the Li –O interaction.Sun et al.[6]prepared a V-doped LiFePO 4/C cathode material through carbothermal reduction.The material showed a high discharge capacity of ~70mAh g ?1at the rate of 20C at room temperature,indicating that high rate charge/discharge capacity is attributed to the effective increase of Li +ion diffusion capability.Hong et al.[7]reported that vanadium ions can be incorporated into the iron-containing olivine structure of LiFePO 4.The 5%V-LiFePO 4material had the highest electronic and ionic conductivities.The high vanadium content reduced the capacity of LiFePO 4.Ma et al.[8]investigated the phase compositions of the LiFePO 4-based cathode materials upon varying the added amount of vanadium,x ,from 0to 0.13.

G.Yang (*):F.Cai

Key Lab for Biomass Gasification Technology of Shandong Province,Energy Research Institute of Shandong Academy of Sciences,Jinan 250014,China

e-mail:yanggai@http://www.77cn.com.cn

G.Yang :C.Jiang :X.He :J.Ying

Institute of Nuclear and New Energy Technology,Tsinghua University,Beijing 100084,China

Ionics (2012)18:59–64

DOI 10.1007/s11581-011-0620-9

Preparation of V -LiFePO 4cathode material for Li-ion batteries

锂电池

The doping of vanadium (x ≤0.07)is beneficial to refine the

particle size and increase the electrical conductivity.Zhao et

al.[9]prepared V-LiFePO 4composites via solid-state

reaction.Both experimental analyses and theoretical simu-

lations show that vanadium does not enter into the LiFePO 4

crystal lattice.

In this study,we attempted to synthesize V-LiFePO 4

cathode material using V-FePO 4precursor by controlled

crystallization –carbothermal process,which has been

experienced as an effective way of mass production for

electrode materials [10].The vanadium content was

investigated from 1%to 5%.The electrochemical perfor-

mance of V-LiFePO 4materials at high rate and low temperature are compared with that of the LiFePO 4material.Experimental In this work,NH 4VO 3,NH 4H 2PO 4,Fe (NO 3)3,H 3PO 4,NH 3H 2O,Li 2CO 3,and sucrose (C 12H 22O 11)were used as starting materials.Firstly,NH 4VO 3with equal mole of NH 4H 2PO 4was dissolved into the solution,which contained Fe (NO 3)3and H 3PO 4with equal molar concentration.With ammonia as the precipitating agent,the pH value was 3–4.Under

these Fig.1SEM images of undoped

FePO 4and V-FePO 460Ionics (2012)18:59–64

锂电池

conditions at 50°C,a light yellow (FePO 4)1?x (VOPO 4)x x H 2O precursor was obtained by a controlled crystallization process [10].The as-obtained precursors were washed with water and dried before further processing.

(FePO 4)1?x (VOPO 4)x ?x H 2O precursors with V doping mole fraction of 1%,3%,and 5%can be obtained by adjusting the amount of NH 4VO 3and NH 4H 2PO 4.The as-obtained precursors were pre-heated at 520°C for 10h to obtain anhydrous V-FePO 4powders.

The molar ratio of V/P was 1:1during the above coprecipitation process.To synthesize V-LiFePO 4cathode materials,the molar ratio of V/P should be in accordance with Li 3V 2(PO 4)3.Therefore,an additional amount of NH 4H 2PO 4should be added to compensate for the amount of P when as-obtained V-FePO 4precursor,Li 2CO 3,C 12H 22O 11,and deionized water (H 2O)were mixed in a mole ratio of Li 2CO 3:FePO 4:C 12H 22O 11:H 2O=0.485:1:0 …… 此处隐藏：12815字，全部文档内容请下载后查看。喜欢就下载吧 ……