李放放,陳仕謀
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高壓鋰離子電池電解液添加劑研究進(jìn)展
李放放,陳仕謀
(中國(guó)科學(xué)院過(guò)程工程研究所,北京 100190)
電壓下的氧化分解限制了高壓鋰離子電池的發(fā)展,為了解決這一問(wèn)題,需要設(shè)計(jì)、合成新型的耐高壓電解液或?qū)ふ液线m的電解液添加劑,然而從經(jīng)濟(jì)效益考慮,發(fā)展合適的電解液添加劑來(lái)穩(wěn)定電極/電解液界面更加受到研究者們的青睞。本文綜述了最近幾年在高壓鋰離子電池電解液添加劑方面的研究進(jìn)展,并按照添加劑的種類(lèi)將其分為6部分進(jìn)行探討:含硼類(lèi)添加劑、有機(jī)磷類(lèi)添加劑、碳酸酯類(lèi)添加劑、含硫添加劑、離子液體添加劑及其它類(lèi)型添加劑。分別對(duì)這些添加劑的作用機(jī)理、作用效果進(jìn)行闡述,展望了添加劑在高壓鋰離子電池中的發(fā)展前景及未來(lái)研究方向。
鋰離子電池;電解液添加劑;高電壓
鋰離子電池由于具備比能量高、循環(huán)壽命長(zhǎng)、無(wú)記憶效應(yīng)等優(yōu)點(diǎn)而引起各研究機(jī)構(gòu)及產(chǎn)業(yè)界的廣泛關(guān)注和深入研究,鋰離子電池技術(shù)取得快速發(fā)展,其應(yīng)用也從3C電子產(chǎn)品、便攜式電子設(shè)備、電動(dòng)工具、電動(dòng)汽車(chē)拓展到規(guī)模儲(chǔ)能、軍工、航空航天等領(lǐng)域[1]。近年來(lái),在世界各國(guó)大力發(fā)展新能源汽車(chē)的背景下,對(duì)鋰離子電池技術(shù)提出了更高要求,因?yàn)樾履茉雌?chē)能否取代燃油車(chē)的關(guān)鍵在于能否突破電池技術(shù)瓶頸,在保證安全的前提下如何提高動(dòng)力電池的能量密度、延長(zhǎng)續(xù)航里程、降低電芯成本是目前國(guó)際上最重要的研究方向。為了提升電池的能量密度,必須開(kāi)發(fā)新的電極材料和電池體系,考慮到鋰硫、鋰空等技術(shù)路線(xiàn)仍需進(jìn)一步完善才能真正走向商業(yè)應(yīng)用,目前開(kāi)發(fā)新一代高性能鋰離子電池體系仍是各大企業(yè)和研究院所的研究重點(diǎn)。綜合文獻(xiàn)報(bào)道和產(chǎn)業(yè)需求來(lái)看[2-4],比較有效的途徑是采用高電壓的正極和高容量的負(fù)極材料,并開(kāi)發(fā)與之相匹配的高壓電解液和高性能隔膜,該類(lèi)電池體系我們簡(jiǎn)稱(chēng)為高壓鋰離子電池。
高壓電解液是構(gòu)筑高壓鋰離子電池體系的核心,因?yàn)樘岣唠姵氐墓ぷ麟妷嚎梢蕴岣吣芰棵芏龋?,目前所使用的電解液?dāng)工作電壓超過(guò)4.3 V時(shí)會(huì)發(fā)生嚴(yán)重的氧化分解[5],導(dǎo)致電極/電解液之間界面阻抗增加,從而惡化電池性能。相比于發(fā)展新型的耐高壓電解液,添加劑由于其用量少、成本低、無(wú)毒或毒性較小等優(yōu)點(diǎn)而更受研究者們的青睞。2006年,ZHANG[6]按照作用機(jī)制的不同,對(duì)添加劑進(jìn)行了系統(tǒng)地分類(lèi)。隨后,關(guān)于添加劑方面的研究逐漸增多,近年來(lái),研究者們進(jìn)一步擴(kuò)大了添加劑的應(yīng)用范圍,開(kāi)始將其應(yīng)用到比能量較高的高壓鋰離子電池中。本綜述主要對(duì)最近幾年應(yīng)用到高壓鋰離子電池中的添加劑進(jìn)行了分類(lèi)總結(jié),并按照添加劑的種類(lèi)將其分為:含硼類(lèi)添加劑;有機(jī)磷類(lèi)添加劑;碳酸酯類(lèi)添加劑;含硫添加劑;離子液體添加劑及其它類(lèi)型添加劑。
[7-9],在電池循環(huán)過(guò)程中,很多含硼化合物會(huì)在正極表面形成保護(hù)膜,來(lái)穩(wěn)定電極/電解液之間的界面,從而提高電池性能??紤]到含硼化合物的這一獨(dú)特性能,眾多學(xué)者開(kāi)始嘗試將其應(yīng)用到高壓鋰離子電池中,來(lái)增強(qiáng)正極界面穩(wěn)定性。LI等[10]將三(三甲基硅烷)硼酸酯(TMSB)應(yīng)用到以L(fǎng)i[Li0.2Mn0.54Ni0.13Co0.13]O2作正極材料的高壓鋰離子電池中,發(fā)現(xiàn)當(dāng)有0.5%(質(zhì)量分?jǐn)?shù))TMSB添加劑存在時(shí),循環(huán)200圈后容量保持74%(電位范圍2~4.8 V,充放電倍率為0.5 C);而沒(méi)有添加劑存在時(shí),容量保持僅為19%。為了解TMSB對(duì)正極表面修飾的作用機(jī)制,ZUO等[11]將TMSB添加到LiNi0.5Co0.2Mn0.3O2/石墨全電池中,并分別對(duì)正極材料進(jìn)行了XPS與TEM分析,得到如圖1所示的結(jié)論:在沒(méi)有添加劑存在時(shí),隨著循環(huán)次數(shù)的增加,會(huì)逐漸在正極表面形成一層有LiF存在的正極電解液界面(CEI)膜,這層膜較厚且具有高阻抗;加入TMSB后,缺電子的含硼類(lèi)化合物會(huì)提高正極表
面LiF的溶解度,形成的SEI膜較薄,且具有低的阻抗。除了TMSB,現(xiàn)如今應(yīng)用到高壓鋰離子電池中的含硼類(lèi)添加劑還包括雙草酸硼酸鋰(LiBOB)[12]、雙氟草酸硼酸鋰(LiFOB)[13]、四甲基硼酸酯 (TMB)[14]、硼酸三甲酯(TB)[15]以及三甲基環(huán)三硼氧烷[16]等,這些添加劑在循環(huán)過(guò)程中會(huì)比電解液溶劑優(yōu)先被氧化,形成的保護(hù)性膜覆蓋到正極表面,這層保護(hù)性膜具有良好的離子導(dǎo)電性,能抑制電解液在隨后的循環(huán)中發(fā)生氧化分解以及正極材料結(jié)構(gòu)的破壞,穩(wěn)定電極/電解液界面,并最終提高高壓鋰離子電池的循環(huán)穩(wěn)定性。
根據(jù)前線(xiàn)軌道能量與電化學(xué)穩(wěn)定性的關(guān)系:分子的HOMO越高,軌道中的電子越不穩(wěn)定,氧化性越好;分子的LUMO越低,越容易得電子,還原性越好。因此,通過(guò)計(jì)算添加劑分子與溶劑分子的前線(xiàn)軌道能量,可以從理論上判斷添加劑的可行性。SONG等[17]利用Gaussian 09程序,采用密度泛函理論(DFT)在B3LYP/6-311+(3df,2p)水平下分別對(duì)三(2, 2, 2-三氟乙基)亞磷酸酯(TFEP)、三苯基亞磷酸酯(TPP)、三(三甲基硅基)亞磷酸酯(TMSP)以及亞磷酸三甲酯(TMP)類(lèi)添加劑以及溶劑分子進(jìn)行優(yōu)化,得到相應(yīng)的優(yōu)勢(shì)構(gòu)象,并對(duì)其進(jìn)行了前線(xiàn)軌道分析,從圖2中可以看出,這些亞磷酸酯化合物的HOMO能量遠(yuǎn)高于溶劑分子,表明亞磷酸酯類(lèi)化合物比溶劑分子具有更高的氧化性,在正極表面能優(yōu)先發(fā)生電化學(xué)氧化,形成SEI膜覆蓋在正極表面。ZHANG等[18]認(rèn)為,對(duì)于亞磷酸酯類(lèi)化合
物,中心的磷(III)原子存在一對(duì)孤對(duì)電子,在含LiPF6的電解液中能與配位,形成的絡(luò)合物可以穩(wěn)定電解液中的鋰鹽。PIRES等[19]系統(tǒng)研究了TFEP在高壓鋰電池中的作用,對(duì)于富鋰正極材料,在首次充電時(shí)會(huì)有氧析出,亞磷酸酯化合物中的磷原子由于還未達(dá)到最高價(jià)態(tài),容易與氧體系(O2、、)反應(yīng),生成可溶的磷酸酯類(lèi)化合物,防止 電解液與氧體系發(fā)生進(jìn)一步反應(yīng),從而穩(wěn)定電池 體系。
除了亞磷酸酯類(lèi)添加劑,目前所用的有機(jī)磷類(lèi)添加劑還包括磷酸酯類(lèi)化合物。XIA等[20]將三烯丙基磷酸酯(TAP)添加劑應(yīng)用到Li[Ni0.42Mn0.42Co0.16]O2(NMC442)/石墨全電池中,發(fā)現(xiàn)當(dāng)有TAP存在時(shí)會(huì)顯著提高庫(kù)侖效率,長(zhǎng)時(shí)間循環(huán)后,仍然具有很高的容量保持,XPS結(jié)果表明,在循環(huán)過(guò)程中,烯丙基可能會(huì)發(fā)生交聯(lián)電聚合反應(yīng),得到的產(chǎn)物覆蓋到電極表面,形成均勻的SEI膜。RONG 等[21]對(duì)比了在有無(wú)三(三甲基硅基)磷酸酯(TMSP)添加劑存在時(shí)LiNi0.4Co0.2Mn0.4O2/石墨全電池的循環(huán)性能,在高電壓下,當(dāng)有TMSP存在時(shí),會(huì)在正極表面形成穩(wěn)定、高導(dǎo)電性的膜,保護(hù)正極材料,抑制過(guò)渡金屬離子的溶解以及電解液的氧化分解。此外,MAI等[22]還嘗試將苯基膦酸二甲酯(DMPP)應(yīng)用到高壓鋰離子電池中,來(lái)增強(qiáng)LiNi0.5Mn1.5O4正極材料在高溫下的循環(huán)穩(wěn)定性,當(dāng)加入DMPP后,在50 ℃下循環(huán)100圈后容量保持高達(dá)91%,而沒(méi)有添加劑存在時(shí),容量保持僅為42%,這主要是因?yàn)镈MPP中氧原子的存在能提高離子導(dǎo)電性,而苯環(huán)的存在對(duì)增強(qiáng)穩(wěn)定性有很大貢獻(xiàn)。
含氟烷基(PFA)化合物具有很高的電化學(xué)穩(wěn)定性,同時(shí)具備疏水性與疏油性的特性,當(dāng)PFA添加到有機(jī)溶劑中,疏溶劑的PFA會(huì)凝聚到一起形成膠團(tuán)[23]。由于PFA的這一特性,2014年,ZHU 等[24]嘗試將全氟烴基(圖3中TEM-EC、PFB-EC、PFH-EC、PFO-EC)取代的碳酸亞乙酯添加到高壓鋰離子電池電解液中,對(duì)于Li1.2Ni0.15Mn0.55Co0.1O2/石墨電池,當(dāng)加入0.5%(質(zhì)量分?jǐn)?shù))的PFO-EC后,電池在長(zhǎng)時(shí)間循環(huán)過(guò)程中性能明顯提高,這主要是因?yàn)樘砑觿┰谘h(huán)過(guò)程中形成了雙層的鈍化膜,同時(shí)減少電極表面的降解與電解液的氧化分解。ZUO等[25]在碳酸亞乙酯上連上乙烯基取代基,得到碳酸乙烯亞乙酯(圖3中VEC)添加劑,并考慮其在LiNi0.4Mn0.4Co0.2O2/石墨全電池中同時(shí)對(duì)正、負(fù)極界面的影響,發(fā)現(xiàn)加入2%(質(zhì)量分?jǐn)?shù))的VEC后,VEC的分解產(chǎn)物能分別在正、負(fù)極形成保護(hù)膜,使界面阻抗降低,循環(huán)穩(wěn)定性增加。除了環(huán)狀碳酸酯類(lèi)添加劑,LEE等[26]還研究了鏈狀氟代碳酸酯作高壓電解液添加劑時(shí)電池的性能及其作用機(jī)制,當(dāng)向電解液中加入5%(質(zhì)量分?jǐn)?shù))的甲基(2, 2, 2-三氟乙基)碳酸酯(圖3中FEMC),在3.0~4.6 V下循環(huán)50圈后容量保持84%,IR與XPS分析結(jié)果顯示,正極表面鈍化膜中含有的金屬氟化物以及C—F鍵組分提高了高壓下的界面穩(wěn)定性。
近年來(lái),將有機(jī)磺酸酯作為添加劑應(yīng)用到鋰離子電池中的報(bào)道很多[27-28]。PIRES等[29]將1, 3-丙磺酸內(nèi)酯(PS)加入到高壓鋰離子電池電解液中,有效抑制了電極表面副反應(yīng)的發(fā)生以及金屬離子的溶解。ZHENG等[30]用二甲磺酰甲烷(DMSM)作為高壓LiNi1/3Co1/3Mn1/3O2/石墨電池電解液添加劑,XPS、SEM以及TEM分析結(jié)果表明,MMDS的存在對(duì)正極SEI膜具有很好的修飾作用,即使在高壓下也能顯著降低電極/電解液界面阻抗,提高正極材料的循環(huán)穩(wěn)定性。此外,HUANG等[31]分別研究了三氟甲基苯硫醚(PTS)添加劑在高壓鋰離子電池室溫及高溫下的循環(huán)性能,理論計(jì)算數(shù)據(jù)與實(shí)驗(yàn)結(jié)果分析得出,在循環(huán)過(guò)程中PTS比溶劑分子優(yōu)先被氧化,形成的SEI膜提高了電池在高電壓下的循環(huán)穩(wěn)定性。此外,一些噻吩[32]及其衍生物[33]也被考慮作為高壓鋰離子電池添加劑使用,當(dāng)加入這些添加劑后,會(huì)在正極表面形成聚合物膜,避免了電解液在高壓下的氧化分解。
離子液體是一種低溫熔融鹽,因其具備蒸汽壓低、電導(dǎo)率高、不易燃、熱穩(wěn)定及電化學(xué)穩(wěn)定性高等優(yōu)點(diǎn)而被廣泛應(yīng)用到鋰離子電池中。目前已報(bào)道的文獻(xiàn)主要是將純離子液體作為普通鋰離子電池電解液使用[34-36],本課題組考慮到離子液體獨(dú)特的物理化學(xué)性質(zhì),嘗試將其作為添加劑應(yīng)用到高壓鋰離子電池中[37],如我們分別將4種烯烴取代咪唑離子液體(圖4)添加到了1.2 mol/L的LiPF6/EC/EMC電解液中,并對(duì)其進(jìn)行了循環(huán)性能測(cè)試,結(jié)果表明,首次充放電效率都明顯提高,尤其添加3%(質(zhì)量分?jǐn)?shù))的[AVIm][TFSI]離子液體時(shí),電池的放電容量和循環(huán)性能最好,特定條件下,電解液甚至可耐4.95 V高壓;通過(guò)XPS及SEM等光譜手段對(duì)SEI膜的組分及表面形貌進(jìn)行了分析,發(fā)現(xiàn)離子液體在LiNi0.5Mn1.5O4材料表面形成一層致密、穩(wěn)定的正極聚合物保護(hù)膜,其組分包括Li2CO3、LiF、LiPF及離子液體聚合物等,從而對(duì)電極起到保護(hù)作用,提高了電池的循環(huán)性能和倍率性能。此外,BAE等[38]用雙(三氟甲基磺酰)亞胺三乙基(2-甲氧乙基)季磷鹽(TEMEP-TFSI)作有
機(jī)電解液添加劑,發(fā)現(xiàn)TEMEP-TFSI可以有效提高Li/LiMn1.5Ni0.5O4半電池的容量保持率,同時(shí)可降低電解液的可燃性。TEM和XPS的結(jié)果表明,添加劑在LNMO表面形成了穩(wěn)定保護(hù)膜,有效抑制了電解液的分解。
除了上面提到的幾種類(lèi)型的添加劑外,CHEN等[39]嘗試用有機(jī)硅類(lèi)化合物作高壓鋰離子電池添加劑,當(dāng)向電解液中加入0.5%(質(zhì)量分?jǐn)?shù))的烯丙氧基三甲硅烷(AMSL)時(shí),電池的循環(huán)性能與熱穩(wěn)定性明顯提高;SEM、XPS及FTIR分析結(jié)果表明,AMSL會(huì)在正極表面形成保護(hù)性膜;另外通過(guò)對(duì)石墨負(fù)極進(jìn)行循環(huán)性能以及CV測(cè)試,發(fā)現(xiàn)加入添加劑后放電容量會(huì)輕微增加,與不含添加劑時(shí)的CV曲線(xiàn)相比,加入AMSL后會(huì)在原來(lái)還原峰相對(duì)較高的電壓處出現(xiàn)一個(gè)新的還原峰,表明AMSL會(huì)優(yōu)先被還原,形成穩(wěn)定的SEI膜覆蓋到石墨負(fù)極的表面,抑制了電解液在電極表面進(jìn)一步的還原分解,增強(qiáng)了循環(huán)穩(wěn)定性,由于A(yíng)MSL能同時(shí)在LiNi0.5Mn1.5O4與石墨負(fù)極形成SEI膜來(lái)穩(wěn)定電極界面,因此其有望成為一種理想的添加劑得到更進(jìn)一步的應(yīng)用。一些苯的衍生物也可用作高壓鋰離子電池添加劑,KANG等[40]將1, 3, 5-羥基苯(THB)加入到碳酸酯類(lèi)電解液中,在高溫、高壓下表現(xiàn)出了良好的熱穩(wěn)定性和電化學(xué)穩(wěn)定性。HUANG等[41]通過(guò)研究發(fā)現(xiàn),4-三氟甲基-苯甲腈(4-TB)添加劑能有效抑制電解液的氧化分解。YANG等[42]在其文獻(xiàn)中指出雙馬來(lái)酰亞胺(BMI)添加劑能提高電解液的穩(wěn)定性,進(jìn)而提高鋰離子電池的高壓循環(huán)性能。TARNOPOLSKIY等[43]對(duì)比了在5 V高壓下,當(dāng)加入不同添加劑后 LiNi0.5Mn1.5O4正極的自放電行為,對(duì)選中的40種添加劑進(jìn)行性能測(cè)試,發(fā)現(xiàn)只有丁二酸酐添加劑能很好地抑制LNMO正極的自放電行為;而其它的添加劑,甚至丁二酸酐的衍生物對(duì)電池的自放電行為均沒(méi)有影響,或者會(huì)產(chǎn)生負(fù)面影響。 SEM和XPS分析結(jié)果表明,向電解液中加入丁二酸酐后會(huì)在電極表面形成穩(wěn)定的SEI膜,使得庫(kù)侖效率明顯提高,降低了循環(huán)過(guò)程中的容量損失。此外,KANG等[44]發(fā)現(xiàn)將5-羥基-1H-吲唑(HI)添加到高壓鋰離子電解液中,電池的循環(huán)穩(wěn)定性明顯提高,這主要是因?yàn)镠I在循環(huán)過(guò)程中比碳酸酯類(lèi)溶劑優(yōu)先氧化,在正極表面形成鈍化膜;但是,HI添加劑的使用會(huì)輕微增加電池極化程度;此外,由于HI的還原電勢(shì)比碳酸酯類(lèi)低,因此添加劑對(duì)石墨負(fù)極材料并沒(méi)有保護(hù)作用。
傳統(tǒng)使用的有機(jī)碳酸酯類(lèi)電解液在高電壓下持續(xù)的氧化分解以及正極材料過(guò)渡金屬離子的溶解問(wèn)題,限制了高壓正極材料的容量發(fā)揮和應(yīng)用,發(fā)展高壓電解液添加劑是改善電池性能既經(jīng)濟(jì)又有效的方法。現(xiàn)今所報(bào)道的高壓添加劑在循環(huán)過(guò)程中一般會(huì)比溶劑分子優(yōu)先氧化,在正極表面形成鈍化膜,穩(wěn)定電極/電解液界面,最終實(shí)現(xiàn)電解液能在高壓下穩(wěn)定存在。從目前公開(kāi)報(bào)道的國(guó)內(nèi)外研究進(jìn)展來(lái)看,在高壓電解液的開(kāi)發(fā)方面,引入高壓添加劑一般可以獲得4.4~4.5 V的電解液。但是對(duì)于富鋰、磷酸釩鋰、高壓鎳錳等正極材料,由于可充電電壓達(dá)到了4.8 V甚至5 V以上,必須開(kāi)發(fā)可耐更高電壓的電解液才能獲得更高的能量密度。本課題組一直致力于開(kāi)發(fā)以離子液體為添加劑或溶劑的電解液,并進(jìn)一步探究其在循環(huán)過(guò)程中的作用機(jī)制,優(yōu)化設(shè)計(jì)合成更加穩(wěn)定、高效的功能化離子液體和電解液新體系。如在中國(guó)科學(xué)院納米先導(dǎo)專(zhuān)項(xiàng)的支持下,我們開(kāi)發(fā)了一系列含醚基、烯基的離子液體并將其應(yīng)用于高壓鎳錳、富鋰高容量正極、硅碳負(fù)極等正負(fù)極材料體系。第三方測(cè)試表明,離子液體電解液可耐4.95 V高壓,2015年電解液組裝成LiNi0.5Mn1.5O4/Li扣式半電池的首效為91.3%,循環(huán)200次后容量保持率為98.3%,Si-C/Li扣式半電池的首效為89.0%,循環(huán)200次后容量保持率為99.5%。由于離子液體具有固有的不易燃性,熱力學(xué)、電化學(xué)穩(wěn)定性以及環(huán)境友好等特點(diǎn),必然成為未來(lái)高壓電解液的一個(gè)重要研究方向。然而由于第一代離子液體為添加劑的電解液的主體仍是碳酸酯類(lèi)有機(jī)溶劑,并沒(méi)有徹底解決電解液的耐高低溫、阻燃等問(wèn)題[45]。為了更好地發(fā)揮離子液體寬溫度適應(yīng)性、耐高壓、高安全性等優(yōu)點(diǎn),本課題組將進(jìn)一步開(kāi)發(fā)以離子液體為部分溶劑甚至全溶劑的第二代離子液體電解液,通過(guò)鋰鹽優(yōu)化、加入有機(jī)助劑及其它功能添加劑等,強(qiáng)化電解液的實(shí)用性,并最終實(shí)現(xiàn)離子液體電解液在高壓鋰離子電池大面積的商業(yè)化應(yīng)用。
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Research progress on electrolyte additives for high voltage lithium-ion batteries
LI Fangfang, CHEN Shimou
(Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China)
The decomposition of traditional electrolytes at high voltage limits the development of high voltage lithium-ion batteries. Approaches to solve these problems include①the design and synthesis of intrinsically stable electrolytes to tolerate high voltages, and②looking for suitable electrolyte additives. The latter approach to stabilize the electrode/electrolyte interphase is particularly attractive from the techno-economic point of view. This review summarizes the latest research progress on electrolyte additives for high voltage lithium-ion batteries. The additives can be divided into six categories: boron-based additives, organophosphorus additives, carbonate additives, sulfur-based additives, ionic liquid additives, and other additives. The effect and working mechanism of the additives are generally described and discussed. The prospect of the high voltage electrolyte additives used in the future were also analyzed.
lithium-ion batteries; electrolyte additives; high voltage
10.12028/j.issn.2095-4239.2016.04.006
O 646.21
A
2095-4239(2016)04-436-07
2016-04-27;修改稿日期:2016-05-30。
國(guó)家自然基金(21276257和91534109),中國(guó)科學(xué)院納米先導(dǎo)專(zhuān)項(xiàng)(XDA09010103)。
李放放(1991—),女,碩士研究生,研究方向?yàn)楦邏轰囯x子電池電解液添加劑,E-mail:ffli@ipe.ac.cn;通訊聯(lián)系人:陳仕謀,研究員,博導(dǎo),研究方向?yàn)殡娀瘜W(xué)、能源化工,E-mail:chenshimou@ipe.ac.cn。