PHEV用高能量密度电池的设计

Design of high energy density

MCMB/Li[Ni1/3Mn1/3Co1/3]O2 cells

for PHEV purposes

Honghe Zheng1,*, Gao Liu*, Xiangyun Song,

Paul Ridgway and Vince Battaglia*, z

Lawrence Berkeley National Laboratory, 1 Cyclotron Rd,

Berkeley, CA 94720, USA

Introduction

Energy density is one of the important criteria for lithium-ion batteries to meet the aggressive requirements for PHEV applications. According to the recently announced PHEV goals by the USABC, a system energy density of 207 Wh/L is required (with an assumption that only 70% is available for all electric driving) to meet the 40-mile, all-electric-driving target. Reducing inactive material content and increasing electrode thickness are important ways to increase the energy density of a lithium-ion battery. We have reported the energy density improvement of the Li[Ni1/3Mn1/3Co1/3]O2 (L333) cathode using minimum amounts of inactive materials[1]. That study investigated the effects of electrode thickness on the electrochemical behavior of graphite anodes and Li[Ni1/3Mn1/3Co1/3]O2-cathodes. In this presentation we show that on the electrode scale combining the optimized MCMB anode and Li[Ni1/3Mn1/3Co1/3]O2 cathode surpasses the PHEV 40-mile energy density goal by 50%. Experiment

MCMB was supplied by Osaka Gas, Japan and Li[Ni1/3Mn1/3Co1/3]O2 was supplied by Seimi, USA. A slurry consisting of different amounts of active material, PVdF, and acetlylene black was prepared by mixing in 1-methyl-2-pyrrolidone (1MP). Coated films on copper foil for MCMB and on aluminum foil for Li[Ni1/3Mn1/3Co1/3]O2 were prepared by the motorized doctor blade method. All of the films with different active material loadings were compressed to 35% porosity using a calendering machine. Coin cells were assembled in an argon-filled glove box. The separator employed was Celgard 2400. 1M LiPF6/EC+DEC(1:2) was used as the electrolyte. Electrochemical measurements were performed by using a Maccor battery cycler.

Results and discussion

Fig.1 shows the effect of electrode thickness on the rate performance for both the MCMB-based anode and the Li[Ni1/3Mn1/3Co1/3]O2-based cathode. From this figure, it is seen that rate performances of the anode and the cathode as a function of the electrode thickness are quite different. The capacity of at which the anode hits the rate 1On leave from Henan Normal University, P.R.China

* Electrochemical active member

z E-mail: VSBattaglia@ mass transfer limit varies dramatically with thicknees and C-rate, where as the capacity of the cathode shows a steady decline as a function of rate before hitting a mass transfer

Fig.2 was obtained by plotting the capacity versus

current density of an electrode corresponding to the

point just before the bend in the curve of the rate-

capability curves of Figure 1. (The performances of

three graphites and L333 are displayed.) This figure

indicates that. for discharge rates below ca. 3 C, the rate

performance of the three graphite anodes is worse than that

of the Li[Ni1/3Mn1/3Co1/3]O2 cathode. In other words, the

anode limits the rate performance of the cell for discharges

longer than 20 minutes. The data also suggest that cells

with discharge rates greater than 3C can not be made with

L333 cathodes. For urban driving, 20 mph is considered the

average driving speed. Therefore, the 40-mile battery

system should be optimized for a 2 hr discharge, i.e. C/2

rate. Based on the data of figure 2, the cycleable capacity of

2

1/31/31/32 full

cells. Cathode contains a: 8% PVdF; b: 2% PVdF.

Fig. 3 shows the power cycling of two

MCMB/Li[Ni1/3Mn1/3Co1/3]O2coin cells we designed for

PHEV purposes. These cells are cycled with a P/4 Charge

to 4.3V, and a P/2 Discharge to 70% depth of discharge

(DOD), with a 1-hour constant voltage hold at the top of

charge. The two cells contain cathodes with different

binder contents, 2% and 8%. The cell with the cathode that

contains 2% binder has an initial useable energy density of

350 Wh/l (volume includes the working area from Al to Cu

current collector). The cell with the cathode that contains

8% binder has an initial useable energy density of 310 Wh/l.

The electrode -based energy density of the both systems

exceeds the PHEV system requirements with excellent

cycling behavior. Meeting the system requirement will

require additional engineering effort. The cells are still

cycling in our laboratory.

Reference

1.Honghe Zheng, Gao Liu, Vince Battaglia et. al, ECS Trans.

11:1-7. 2008

Li[Ni1/3Mn1/3Co1/3]O2 cathode (b) of different

thicknesses.