山(shan)東埃爾派 | 點擊量(liang):0次(ci) | 2019-11-30
【鋰電測量】深度解析鋰電池電化學測量方法
鋰電池的(de)電極(ji)反應主要包括哪些
電(dian)(dian)(dian)池中(zhong)電(dian)(dian)(dian)極過(guo)(guo)程(cheng)(cheng)一般包(bao)括溶液相中(zhong)離(li)子(zi)的傳(chuan)輸,電(dian)(dian)(dian)極中(zhong)離(li)子(zi)的傳(chuan)輸,電(dian)(dian)(dian)極中(zhong)電(dian)(dian)(dian)子(zi)的傳(chuan)導,電(dian)(dian)(dian)荷(he)轉(zhuan)移,雙電(dian)(dian)(dian)層或(huo)空問電(dian)(dian)(dian)荷(he)層充放電(dian)(dian)(dian),溶劑、電(dian)(dian)(dian)解質中(zhong)陰陽(yang)離(li)子(zi),氣相反應物或(huo)產物的吸附(fu)脫附(fu),新相成核長(chang)大,與電(dian)(dian)(dian)化(hua)學反應耦合的化(hua)學反應,體積變化(hua),吸放熱(re)等過(guo)(guo)程(cheng)(cheng)。這些過(guo)(guo)程(cheng)(cheng)有些同時進行,有些先(xian)后發生。
電極過程的驅動(dong)力包括(kuo)電化學(xue)勢、化學(xue)勢、濃度(du)梯度(du)、電場梯度(du)、溫度(du)梯度(du)。
分清兩電(dian)極和三電(dian)極
電(dian)化學測量一般采(cai)用兩電(dian)極電(dian)池(chi)或三電(dian)極電(dian)池(chi),較少(shao)使用四電(dian)極電(dian)池(chi)。
兩電極
兩(liang)電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)由研究電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)(W),亦(yi)稱之(zhi)為工作電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)和輔助(zhu)電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)(C),亦(yi)稱之(zhi)為對電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)組成。鋰電(dian)(dian)(dian)池的(de)研究中多數為兩(liang)電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)電(dian)(dian)(dian)池,兩(liang)電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)電(dian)(dian)(dian)池測量的(de)電(dian)(dian)(dian)壓是正極(ji)(ji)(ji)(ji)電(dian)(dian)(dian)勢與負(fu)極(ji)(ji)(ji)(ji)電(dian)(dian)(dian)勢之(zhi)差,無法單(dan)獨獲(huo)得其中正極(ji)(ji)(ji)(ji)或負(fu)極(ji)(ji)(ji)(ji)的(de)電(dian)(dian)(dian)勢及其電(dian)(dian)(dian)極(ji)(ji)(ji)(ji)過(guo)程動力學信息。
三電極
三(san)電(dian)(dian)(dian)(dian)極電(dian)(dian)(dian)(dian)池包(bao)括,W和(he)C分別(bie)是工作電(dian)(dian)(dian)(dian)極和(he)對電(dian)(dian)(dian)(dian)極,R是參比(bi)電(dian)(dian)(dian)(dian)極。W和(he)C之間通(tong)過極化(hua)(hua)電(dian)(dian)(dian)(dian)流(liu),實現電(dian)(dian)(dian)(dian)極的(de)極化(hua)(hua)。W和(he)R之間通(tong)過極小的(de)電(dian)(dian)(dian)(dian)流(liu),用于測(ce)量工作電(dian)(dian)(dian)(dian)極的(de)電(dian)(dian)(dian)(dian)勢。
通過三電(dian)極電(dian)池,可以專門(men)研究(jiu)工作電(dian)極的(de)電(dian)極過程(cheng)動(dong)力學(xue)。
參比電極的特征
1、參比電極應(ying)為可逆電極:
2、不(bu)易被(bei)極化,以保證電極電勢 比較標準和(he)恒定:
3、具有較好(hao)的恢復特性,不發生 嚴重的滯后現象;
4、具有(you)較好的穩定性和重現(xian)性;
5、快速暫態測量(liang)時,要求參(can)比電極具有較低的(de)(de)電阻(zu),以減少干擾(rao),提(ti)高測量(liang)系(xi)統的(de)(de)穩定性;
6、不同的(de)溶液(ye)(ye)體系,采用相(xiang)同的(de)參比(bi)電極的(de),其測量結(jie)果可能存在差異,誤差主要來源于溶液(ye)(ye)體系間(jian)的(de)相(xiang)互污染和液(ye)(ye)接界電勢(shi)的(de)差異。
常用的參比電極
水溶液體系參比(bi)電(dian)極(ji):可逆氫(qing)電(dian)極(ji)、甘汞(gong)(gong)(gong)電(dian)極(ji)、汞(gong)(gong)(gong)一氧化汞(gong)(gong)(gong)電(dian)極(ji)、汞(gong)(gong)(gong)一硫酸亞汞(gong)(gong)(gong)電(dian)極(ji)等;
非水溶液體系(xi)參比電(dian)(dian)(dian)極:銀(yin)一氯化銀(yin)電(dian)(dian)(dian)極、Pt 電(dian)(dian)(dian)極以及金屬鋰、鈉等電(dian)(dian)(dian)極。
其他:也可(ke)以用銀絲(si)、鉑絲(si)做準參比電(dian)(dian)極,或者(zhe)采用電(dian)(dian)化(hua)學(xue)反應(ying)電(dian)(dian)位穩定的溶解于電(dian)(dian)解液的二茂(mao)鐵氧化(hua)還原(yuan)電(dian)(dian)對(dui)。
關于準參比電極細節(jie)可參考(kao)A.J.Bard編著的(Electrochemical Methods)。
電極過程
電(dian)極過程一般情況下(xia)包括下(xia)列(lie)基本過程或步驟:
1、電(dian)(dian)化(hua)學反(fan)應過(guo)程(cheng)(cheng):在電(dian)(dian)極/溶液(ye)界面上得到或 失去(qu)電(dian)(dian)子(zi)生成反(fan)應產(chan)物的過(guo)程(cheng)(cheng),即電(dian)(dian)荷(he)轉(zhuan)移過(guo)程(cheng)(cheng);
2、傳質過(guo)程:反(fan)應物向(xiang)電(dian)極表面(mian)或(huo)內部傳遞(di)或(huo)反(fan)應產物自電(dian)極內部或(huo)表面(mian)向(xiang)溶液中或(huo)向(xiang)電(dian)極內部的傳遞(di)過(guo)程(擴散和遷移);
3、電極界面處靠近電解液一(yi) 側的(de)雙電層(ceng)以及靠近電極內一(yi)側的(de)空(kong)間(jian)電荷(he)層(ceng)的(de)充放電過程;
4、溶(rong)液中離子的(de)電遷移或電子導(dao)體、電 極內電子的(de)導(dao)電過程。
此(ci)外,伴(ban)隨(sui)電化學反應,還有(you)溶劑(ji)、陰陽(yang)離子、電化學反應產(chan)物的吸附(fu)/脫附(fu)過程,新相生長(chang)過程以及(ji)其它化學反應等。
典型的電極過程及(ji)動力學(xue)參數
離(li)(li)子(zi)(zi)(zi)在電(dian)(dian)(dian)(dian)解質中(zhong)的(de)遷移電(dian)(dian)(dian)(dian)阻(zu)(Rsol);離(li)(li)子(zi)(zi)(zi)在電(dian)(dian)(dian)(dian)極(ji)表面的(de)吸(xi)附電(dian)(dian)(dian)(dian)阻(zu)和(he)電(dian)(dian)(dian)(dian)容(rong)(rong)(Rad,Cad);電(dian)(dian)(dian)(dian)化(hua)學雙電(dian)(dian)(dian)(dian)層電(dian)(dian)(dian)(dian)容(rong)(rong)(Cdl);空間電(dian)(dian)(dian)(dian)荷層電(dian)(dian)(dian)(dian)容(rong)(rong)(Csc);離(li)(li)子(zi)(zi)(zi)在電(dian)(dian)(dian)(dian)極(ji)電(dian)(dian)(dian)(dian)解質界面的(de)傳輸(shu)電(dian)(dian)(dian)(dian)阻(zu)(Rincorporation);離(li)(li)子(zi)(zi)(zi)在表面膜中(zhong)的(de)輸(shu)運(yun)電(dian)(dian)(dian)(dian)阻(zu)和(he)電(dian)(dian)(dian)(dian)容(rong)(rong)(Rfilm,Cfilm);電(dian)(dian)(dian)(dian)荷轉移(Rct);電(dian)(dian)(dian)(dian)解質中(zhong)離(li)(li)子(zi)(zi)(zi)的(de)擴(kuo)散電(dian)(dian)(dian)(dian)阻(zu) (Zdiffusion);電(dian)(dian)(dian)(dian)極(ji)中(zhong)離(li)(li)子(zi)(zi)(zi)的(de)擴(kuo)散(Zdiffusion)——體相擴(kuo)散(Rb)和(he)晶粒晶界中(zhong)的(de)擴(kuo)散(Rgb);宿(su)主晶格中(zhong)外來原子(zi)(zi)(zi)/離(li)(li)子(zi)(zi)(zi)的(de)存儲電(dian)(dian)(dian)(dian)容(rong)(rong)(Cchem);相轉 變反應電(dian)(dian)(dian)(dian)容(rong)(rong)(Cchem);電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)的(de)輸(shu)運(yun)(Re)。
值得注意的是(shi),不(bu)同電(dian)極(ji)過(guo)程的響應(ying)時間不(bu)同,總的來說:電(dian)荷轉移<表(biao)面反(fan)應(ying)<電(dian)子傳輸<界面擴(kuo)散(san)<固(gu)(gu)相反(fan)應(ying)<體相擴(kuo)散(san),因此離子在電(dian)極(ji)、電(dian)解(jie)質材料內部的擴(kuo)散(san)以(yi)及固(gu)(gu)相反(fan)應(ying)一(yi)般是(shi)速率控制步驟。
極化及類型
在施加了外來電(dian)(dian)場后,電(dian)(dian)池或電(dian)(dian)極逐(zhu)漸(jian)偏離平衡電(dian)(dian)勢的狀態(tai),稱之為極化。極化電(dian)(dian)勢與平衡電(dian)(dian)勢的差值的大小被稱之為過電(dian)(dian)勢。
在(zai)不具(ju)有流(liu)動相(xiang)的電池中,存在(zai)著3種類型(xing)的極化:
1、化(hua)學(xue)(xue)極(ji)化(hua)—— 與電(dian)(dian)(dian)(dian)荷(he)轉移過程有關(guan)的(de)極(ji)化(hua),極(ji)化(hua)的(de)驅動力是(shi)電(dian)(dian)(dian)(dian)場梯(ti)度(du)(du)。電(dian)(dian)(dian)(dian)化(hua)學(xue)(xue)極(ji)化(hua)的(de)大小(xiao)是(shi)由電(dian)(dian)(dian)(dian)化(hua)學(xue)(xue)反應速(su)率決定的(de),電(dian)(dian)(dian)(dian)化(hua)學(xue)(xue)極(ji)化(hua)電(dian)(dian)(dian)(dian)阻(Rct)的(de)大小(xiao)與交換(huan)電(dian)(dian)(dian)(dian)流(liu)密度(du)(du)直接相關(guan)。
2、濃(nong)(nong)差極(ji)(ji)(ji)化(hua)(hua)——與參與電(dian)化(hua)(hua)學反應的(de)反應物和產物的(de)擴(kuo)散過程(cheng)有關(guan)的(de)極(ji)(ji)(ji)化(hua)(hua),極(ji)(ji)(ji)化(hua)(hua)的(de)驅動力為濃(nong)(nong)度梯度。濃(nong)(nong)差極(ji)(ji)(ji)化(hua)(hua)與傳質(zhi)粒(li)子的(de)擴(kuo)散系數有關(guan)。
3、歐(ou)(ou)姆極(ji)化(hua)——與(yu)載流子在電(dian)(dian)池中(zhong)各(ge)相輸運(yun)有關的(de)極(ji)化(hua),驅動力(li)是電(dian)(dian)場(chang)梯度。歐(ou)(ou)姆極(ji)化(hua)的(de)大小是由(you)電(dian)(dian)池內部涉及到電(dian)(dian)遷 移(yi)的(de)各(ge)類電(dian)(dian)阻之和(he),即歐(ou)(ou)姆電(dian)(dian)阻決定的(de)。
若(ruo)還(huan)存在其它基(ji)本(ben)電極(ji)過程(cheng)(cheng),如勻(yun)相或多相化(hua) 學反應過程(cheng)(cheng),則可能存在化(hua)學反應極(ji)化(hua)。
電(dian)化學測量(liang)方法在鋰電(dian)池中的經(jing)典應用
鋰離子電(dian)池(chi)電(dian)極材料在電(dian)池(chi)充放(fang)電(dian)過程中一般經歷以下幾個(ge)步驟:
1、劑化的鋰離子(zi)從電解液(ye)內 遷移到電解液(ye)/固體電極(ji)的兩相界面;
2、溶劑化的鋰 離(li)子吸(xi)附(fu)在(zai)電(dian)解(jie)液/固體電(dian)極(ji)的兩(liang)相界面;
3、去溶劑化;
4、電(dian)(dian)荷轉(zhuan)移(yi),電(dian)(dian)子注入電(dian)(dian)極材料的導(dao)帶,吸附態的鋰(li)離(li)子從(cong)電(dian)(dian)解液相遷移(yi)至活(huo)性材料表面晶格;
5、鋰離子(zi)從活性材料表面(mian)晶格向內部(bu)擴散或遷移;
6、電子從集流(liu)體向活性材料的遷移。
循環伏安法
常(chang)(chang)用于電(dian)(dian)極 反應的可逆性、電(dian)(dian)極反應機理(如中間體、相(xiang)界(jie)吸 /脫(tuo)附(fu)、新相(xiang)生成、偶聯化學反應的性質等)及電(dian)(dian)極反應動力學參數(shu)(如擴散系數(shu)、電(dian)(dian)極反應速(su)率常(chang)(chang)數(shu)等)的探(tan)究。
電勢向陰極(ji)方(fang)向掃(sao)描時,電活性物(wu)質在電極(ji)上還(huan)原(yuan),產生還(huan)原(yuan)峰;向陽極(ji)方(fang)向掃(sao)描,還(huan)原(yuan)產物(wu)重(zhong)新(xin)在電極(ji)上氧化(hua)(hua),產生氧化(hua)(hua)峰。因而一(yi)次掃(sao)描,完成一(yi)個還(huan)原(yuan)和氧化(hua)(hua)過程的循環,其電流(liu)一(yi)電壓曲(qu)線(xian)(xian)稱(cheng)為循環伏安曲(qu)線(xian)(xian)。
通過(guo)(guo)循環伏(fu)安曲線的氧化峰(feng)(feng)和(he)還(huan)原峰(feng)(feng)的峰(feng)(feng)高、對稱(cheng)性、氧化峰(feng)(feng)與還(huan)原峰(feng)(feng)的距離,中點位置(zhi),可(ke)判斷電活性物(wu)質在電極表面(mian)反(fan)應(ying)的可(ke)逆程度和(he)極化程度。如果氧化與還(huan)原反(fan)應(ying)的過(guo)(guo)電位 差別不大的化,還(huan)可(ke)以把一(yi)對氧化峰(feng)(feng)與還(huan)原峰(feng)(feng)之間的中點值近似作為該反(fan)應(ying)的熱力學平衡(heng)電位值。
另外恒(heng)電流充放電的電壓(ya)容量曲線微(wei)分后(hou)以(yi)dQ/dV 作為縱軸,橫軸為電壓(ya),可(ke)以(yi)獲得與CV曲線十分相似的結(jie)果,其實(shi)本質并沒區別。
循環伏安法計算鋰離子(zi)擴散系數
注意:僅適用于擴散過程(cheng)為控制步驟(zou)且電(dian)極為可逆體(ti)系,此時有公式(shi)
常溫時,
式中(zhong),Ip為(wei)峰電流(liu)的大小,n為(wei)參與反應的電子數,A為(wei)浸入溶(rong)液中(zhong)的電極(ji)面(mian)積(ji),F為(wei)法(fa)拉(la)第常(chang)數,DLi為(wei)Li在電極(ji)中(zhong)的擴散(san)系數,v為(wei)掃(sao)描速(su)率,△C0為(wei)反應前后(hou)待測濃度的變化。
可以按如下步驟來計算(suan):
1、測量電極(ji)材料在不同掃描速率(lv)下的(de)循環伏(fu)安曲線;
2、將不同掃(sao)描速率下的峰值電流對(dui)掃(sao)描速率的平方根作圖;
3、對峰值電(dian)流(liu)進(jin)行積分,測量(liang)樣品(pin)中鋰的濃度變化;
4、將(jiang)相關參數帶(dai)入式(2),即(ji)可(ke)求得擴(kuo)散系數。
需要注意的(de)是,由于以下幾個原因,其(qi)測得的(de)絕對值在不同(tong)文獻中不盡(jin)相同(tong)
需(xu)要該反應收擴(kuo)散(san)控制,而且(qie)循環(huan)伏安測到的化學擴(kuo)散(san)系數并(bing)非電(dian)極材料(liao)內(nei)部本征(zheng)的離(li)子擴(kuo)散(san)系數(詳細內(nei)容請見擴(kuo)展閱讀)。還有就是(shi),如果是(shi)多孔粉末電(dian)極,其真實反應面積遠大于電(dian)極幾何面積,且(qie)難以精確測量,給結果帶來很大的不確定性。
當然,關于循環伏安法原理使用等(deng),我們會另外詳細介(jie)紹。
恒(heng)電流(liu)間歇滴(di)定技術
恒電(dian)(dian)流(liu)問(wen)歇滴(di)定技術(galvanostatic intermittent titration technique)由德國科學家W.Weppner提出, 基(ji)本原理是在某一特定環境(jing)下對(dui)測量(liang)體系施加一恒 定電(dian)(dian)流(liu)并(bing)持(chi)續(xu)一段時(shi)問(wen)后切斷該電(dian)(dian)流(liu),觀(guan)察施加電(dian)(dian)流(liu)段體系電(dian)(dian)位(wei)隨時(shi)間的變化以及(ji)弛(chi)豫后達到平衡的電(dian)(dian)壓,通過分(fen)析電(dian)(dian)位(wei)隨時(shi)閭的變化可以得出電(dian)(dian)極過 程過電(dian)(dian)位(wei)的弛(chi)豫信(xin)息,進而推測和計算反應(ying)動力(li)學信(xin)息。
當(dang)體系滿(man)足如下條件時,可以計算鋰離子擴(kuo)散系數
1、電極體系(xi)為等溫(wen)絕熱(re)體系(xi);
2、電極體系在(zai)施加電流時無(wu)體積變(bian)化(hua)與(yu)相變(bian);
3、電(dian)極(ji)響應完全由離(li)子在電(dian)極(ji)內部的(de)擴散控(kong)制;
4、τ≤L2/D,L為離(li)子擴(kuo)散(san)長(chang)度;
5、電極材料的電子(zi) 電導遠(yuan)大于離子(zi)電導等條(tiao)件
計算公式如下:
式中,DLi為(wei)(wei)“在電(dian)極(ji)中的(de)(de)(de)化學(xue)擴散系數,Vm為(wei)(wei)活性物質的(de)(de)(de)體積,A為(wei)(wei)浸(jin)入溶液中的(de)(de)(de)真實電(dian)極(ji)面(mian)積,F為(wei)(wei)法(fa)拉第常數,n為(wei)(wei)參與反(fan)應的(de)(de)(de)電(dian)子(zi)數目,I0為(wei)(wei)滴(di)定電(dian)流(liu)值(zhi),dE/dx為(wei)(wei)開路電(dian)位(wei)對(dui)電(dian)極(ji)中Li濃(nong)(nong)度(du)曲線(xian)上某濃(nong)(nong)度(du)處的(de)(de)(de)斜(xie)率(即庫侖滴(di)定曲線(xian)),dE/dt1/2為(wei)(wei)極(ji)化電(dian)壓對(dui)時間平方根(gen)曲線(xian)的(de)(de)(de)斜(xie)率。
利用GITT方法(fa)測量(liang)電(dian)(dian)極(ji)材料中的(de)鋰化學擴散系(xi)(xi)數(shu)基本過(guo)程如下:在電(dian)(dian)池(chi)充放電(dian)(dian)過(guo)程中的(de)某一 時(shi)刻,施加微小電(dian)(dian)流并恒(heng)定(ding)一段時(shi)間后切(qie)斷;記錄電(dian)(dian)流切(qie)斷后的(de)電(dian)(dian)極(ji)電(dian)(dian)位(wei)隨時(shi)間的(de)變(bian)化;做出極(ji)化電(dian)(dian)壓對時(shi)間平方根曲(qu)線,即(ji)dE/dt1/2曲(qu)線;測量(liang)庫侖滴定(ding)曲(qu)線,即(ji)dE/dx曲(qu)線;代入(ru)相關參(can)數(shu),利用公式(shi)求(qiu)解擴散系(xi)(xi)數(shu)。
恒電勢(shi)間歇滴定技(ji)術(shu)
恒電(dian)(dian)位(wei)間(jian)(jian)歇(xie)滴(di)定技術(potentiostatic intermittent titration technique)是(shi)通過瞬時(shi)(shi)改變(bian)電(dian)(dian)極電(dian)(dian)位(wei)并(bing)恒定 該電(dian)(dian)位(wei)值,同時(shi)(shi)記錄電(dian)(dian)流隨(sui)時(shi)(shi)間(jian)(jian)變(bian)化的測量方法。通過分析(xi)電(dian)(dian)流隨(sui)時(shi)(shi)間(jian)(jian)的變(bian)化可(ke)以得出電(dian)(dian)極過程電(dian)(dian)位(wei)弛豫信息以及其它動力學信息, 類似(si)于恒電(dian)(dian)位(wei)階 躍,只是(shi)PITT是(shi)多電(dian)(dian)位(wei)點(dian)測量。
使用(yong)恒電位間(jian)歇(xie)滴定(ding)技術測量鋰(li)離子化學擴散系數(shu)公式(shi)如下:
式中(zhong),i為電流(liu)值,t為時間,△Q為嵌入電極(ji)的電量,DLi為Li在(zai)電極(ji)中(zhong)的擴(kuo)散(san)系數,d為活性物質的厚度。
基本操作如(ru)下:以(yi)恒定電(dian)位(wei)步長(chang)瞬間改(gai)變(bian)電(dian) 極(ji)電(dian)位(wei),記錄電(dian)流隨時間的變(bian)化;利用(yong)方程(8)做(zuo) 出(chu)ln(i)一t曲線;截取ln(i)一t曲線線性(xing)部(bu)分的數據,求(qiu)斜率即可求(qiu)出(chu)鋰離子(zi)化學擴散系數。
電位弛豫技術
電(dian)(dian)位弛豫技術(potential relax technique)是在電(dian)(dian)池與外(wai)界無物質和能量交換的(de)(de)條件下(xia)研究(jiu)電(dian)(dian)極電(dian)(dian) 勢隨時間的(de)(de)變化(hua)關系,該方法(fa)(fa)(fa)屬于電(dian)(dian)流階躍測(ce)量方 法(fa)(fa)(fa)中的(de)(de)斷(duan)電(dian)(dian)流法(fa)(fa)(fa),與GITT實(shi)驗方法(fa)(fa)(fa)一(yi)致,不(bu)同的(de)(de)是分析弛豫過程中的(de)(de)電(dian)(dian)位變化(hua)。該方法(fa)(fa)(fa)最早由中國(guo)科學院物理研究(jiu)所王慶(qing)等運用于鋰離子電(dian)(dian)池電(dian)(dian)極材料中的(de)(de)離子擴散動力學研究(jiu)。
計算公式為:
式中(zhong)(zhong),ψm為(wei)(wei)(wei)平(ping)衡電極電位,ψ為(wei)(wei)(wei)初(chu)始電位,R 為(wei)(wei)(wei)氣體(ti)常數,T為(wei)(wei)(wei)絕對溫度,d為(wei)(wei)(wei)活性物質的厚度,DLi為(wei)(wei)(wei)Li在電極中(zhong)(zhong)的擴散系數,t 為(wei)(wei)(wei)電位達到平(ping)衡時的時間。
具體測量步驟如下:對(dui)電(dian)(dian)(dian)(dian)池預充放電(dian)(dian)(dian)(dian),使電(dian)(dian)(dian)(dian)池的庫侖(lun)效率(lv)降至(zhi)97%左右;在電(dian)(dian)(dian)(dian)池充/放電(dian)(dian)(dian)(dian)到一(yi) 定程度(du)時,切斷電(dian)(dian)(dian)(dian)流,采用(yong)CPT(chrono potentiometry technique)記(ji)錄電(dian)(dian)(dian)(dian)壓隨時問的變化曲(qu)(qu)線(xian);運用(yong)公(gong)式對(dui)ln[exp(ψm-ψ)xF/RT-1]-t作(zuo)圖(tu),并(bing)對(dui)后(hou)半部(bu)分作(zuo)線(xian)性擬(ni)合;ln[exp(ψm-ψ)xF/RT-1]-t曲(qu)(qu)線(xian)進行(xing)擬(ni)合,求解擬(ni)合曲(qu)(qu)線(xian)斜率(lv),帶(dai)公(gong)式即(ji)可求得(de)鋰的化學擴散系數。
運用電(dian)位弛豫技術測(ce)量電(dian)極過程動力(li)學(xue)信息 需要滿足一(yi)定的(de)(de)(de)前提條件。通常(chang),鋰離(li)子電(dian)池(chi)在首(shou) 周充(chong)放(fang)(fang)電(dian)過程中伴(ban)隨著一(yi)些(xie)副反(fan)(fan)應(ying),典(dian)型的(de)(de)(de)副反(fan)(fan)應(ying) 為(wei)SEI膜的(de)(de)(de)形(xing)成(cheng),為(wei)避免(mian)副反(fan)(fan)應(ying)的(de)(de)(de)發生對鋰離(li)子化 學(xue)擴(kuo)散系(xi)數測(ce)量所(suo)(suo)帶來的(de)(de)(de)干擾(rao),通常(chang)電(dian)池(chi)需要進(jin)行 幾個充(chong)放(fang)(fang)電(dian)循環之(zhi)后開(kai)始測(ce)量其化學(xue)擴(kuo)散系(xi)數。此 外(wai),由于(yu)電(dian)位弛豫是一(yi)個非常(chang)緩慢的(de)(de)(de)過程,一(yi)般在 8 h左右,在經過長時間的(de)(de)(de)弛豫后,電(dian)位仍(reng)不能(neng)達到平衡狀態,則有可能(neng)是儀(yi)器(qi)漏電(dian)所(suo)(suo)造成(cheng),需要特別注(zhu)意。
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