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Materials Sciences and Applications, 2018, 9, 155-173

http://www.scirp.org/journal/msa

ISSN Online: 2153-1188

ISSN Print: 2153-117X

DOI:

10.4236/msa.2018.91011 Jan. 24, 2018 155 Materials Sciences and Applications

Ageing Behavior of LiNi

0.80

0.15

0.05

Cathode Based Lithium Ion Cells—Influence

of Phase Transition Processes

Christopher Betzin

1,2

, Holger Wolfschmidt

Siemens AG, Erlangen, Germany

Friedrich-Alexander-University, Erlangen, Germany

Abstract

In this paper,

commercial lithium ion battery cells consisting of graphite

based anode and LiNi

0.80

0.15

0.05

(NCA) oxide based cathode were inve

tigated regarding their aging behavior. The capacity loss is dependent on the

state of charge (SOC) whereas the battery is operated with partial cycles at d

fined SOCs. The structural change of the positive electrode material is ident

fied as dominating aging process. Especially grid services such as primary

control reserve are of economic interest for battery sy

stem operators. In this

application,

small charge and discharge cycles are the main operation mode.

Considering the operation of single battery storage systems in a virtual storage

power plant, different states of charges are much more of interest. Thus th

battery aging behavior of lithium ion ce

lls with NCA based cathode material

with respect to cycling at specific state of charge with small depth of discharge

(DOD) is investigated. The results in this paper provide understanding of

small DOD cycling at given SOC behavior which is necessary for NCA lif

time prediction in this particular case, especially

in virtual storage plant with

various storage systems and thus various SOCs for delivery primary reserve

the small DOD behavior which has an important impa

ct on efficiency and

economy. It is a key finding,

which aging mechanisms are essential in order to

optimize the cell operation and adapt it to system performance.

Keywords

Cyclic Voltammetry, NCA, Fatigue of LiNCAO, Primary Control Reserve

1. Introduction

Lithium ion batteries play a prominent role for energy storage and are an increa-

How to cite this paper:

Betzin, C. a

Wolfschmidt

, H. (2018)

Ageing Behavior of

LiNi

0.80

0.15

0.05

Cathode Based Li-

hium Ion Cells—Influence of Phase Transi-

tion Processes

Materials Sciences and Appli

cations

, 155-173.

https://doi.org/10.4236/msa.2018.91011

Received:

October 24, 2017

Accepted:

January 21, 2018

Published:

January 24, 2018

8 by authors and

Scientific

Research Publishing Inc.

This work is licensed under the Creative

Commons Attribution International

License (CC BY

4.0).

http://creativecommons.org/licenses/by/4.0/

Open Access

C. Betzin, H. Wolfschmidt

DOI:

10.4236/msa.2018.91011 156 Materials Sciences and Applications

singly important part of storage technologies in different sectors [1]-[6]. Based

on the reasonable power and energy density of lithium ion batteries, they occupy

a huge market share on consumer market like portable electronic devices (e.g.

notebooks and tablets) [7]. In the transportation sector, lithium ion storage is at

the moment almost indispensable. For full electric vehicles, lithium ion batteries

are more important than ever before [8]. Also in hybrid vehicles, lithium ion

batteries are used [9]. In combination with grid support, battery vehicles could

even supply primary control reserve [10].

But the key role for grid support in an energy market with high share of re-

newable energy behooves the stationary lithium ion battery energy storage ex-

cluded pumped hydro storage [11]. In the case of primary control reserve, li-

thium ion battery systems are the most promising technology, because of their

efficiency and cost structure [12]. One promising use case of lithium ion battery

storage systems could be the combination of residential battery systems, to store

renewable energy, and the supply of primary control reserve in a virtual storage

power plant [13]. For primary control reserve, there are no more full cycles for

relevance, but partial cycling will gain in importance. In this case, it is insuffi-

cient to know what the impact of decreasing depth of discharge (DOD) is, but

even more it is necessary to know in which state of charge region the system be-

havior could be improved considering aging characteristics of lithium ion cells.

The high efficiency of lithium ion cells compared to other storage technolo-

gies like redox flow techniques or lead acid is an advantage. But, it is also cell

degradation via aging observed, which is the main disadvantage of lithium ion

batteries. An overview of aging mechanisms in literature could be found in [14].

The main aging mechanisms could be divided into two parts, the aging behavior

on cycling and the calendric aging effects [15], both effects occur on anode elec-

trodes as well as on cathode electrodes. The most important anode material is

graphite. The aging of graphite is dominated by growth of the solid electrolyte

interface (SEI) [16] which leads to an increase of internal resistance. It changes

the electrode/electrolyte interface via building a solid inorganic electrolyte inter-

face. The main mechanism is the decomposition of organic electrolyte. SEI

growth with direct increasing of internal resistance is the main factor of calen-

dric aging. In addition different mechanisms on the cathode side are known for

calendric aging. In literature, different materials with different aging behaviors

are known, especially deactivation of active material, dissolution of particles and

side-reactions of active material [17] [18] [19].

As mentioned before, all active materials suffer under degradation by cycling

[20] [21] [22] via mechanical stress and specific chemical depending mechan-

isms. Based on volumetric change of active material and dissolution and growth

of SEI micro, cracks could be observed which are leading to lithium plating [23].

Common used cathode electrode materials are the olivine structured LiFePO

manganese spinel electrode LiMn

(LMO) and layered electrodes like

1+x

(Ni

1/3

)

1−x

(NMC). One capacity loss factor of LFP based cathode

is dissolution of Fe

caused due to impurities, which could be generated at sur-

C. Betzin, H. Wolfschmidt

DOI:

10.4236/msa.2018.91011 157 Materials Sciences and Applications

face while cycling [24]. In addition, at the LFP cathode, cracks could be observed

by anisotropic effects during coexistence of a Li-rich and a Li-poor phase while

cycling [25]. Positive electrodes with LiMn

(LMO) spinel underlay a degrada-

tion caused by local structure effects. The mechanism is the conversion of cubic

LiMn

to a new tetragonal spinel phase with following disintegration in the

orthorhombic LiMn

phase [26]. The layered NMC is very stable by cycling,

but structural rearrangements of NMC and change of stochiometry of transition

metals leads to capacity loss caused due to higher electrolyte reactivity, which

promotes side reactions [27].

In this study the focus is on the cathode material NCA, which is also a layered

oxide material and known for long term stability and high energy density. The

calendric aging progress is minted on the main effect of a graphite/NCA cell via

the growth of SEI and is described in [28]. But changing from calendaric aging

to the degradation on cycling aging a strong influence of the cathode is found in

[29]. In another investigation on NCA based lithium ion batteries increased

current rates with high DOD lead to a rapid degradation of the active material

caused by micro cracks on the surface [30]. Especially this finding will be con-

ducted in the following discussion for the findings obtained here. In addition an

inactive state of Ni

and Ni

is observed in [31] with oxygen loss of the active

material. This leads to disabled lithium for intercalation and deintercalation, re-

sulting in changes of the cell behavior at exactly given potentials. As an impor-

tant effect in NCA materials a phase transition is known with a crystal structure

change between monoclinic to hexagonal [32]. This transition is at a fixed po-

tential which can directly correlated with the finding obtained in the results part

and discussed later on.

2. Measurements

In this section the measurement procedures are presented. The investigated cells

are commercial 45 Ah lithium ion battery round cells with a graphite based

anode and a LiNi

0.80

0.15

0.05

(NCA) based cathode material with organic

electrolyte and hexafluorophosphate lithium salt LiPF

. The procedure is started

with a galvanostatic measurement for cycling with small depth of discharges at

specific state of charge (SOC) and subsequent periodic capacity determination to

investigate capacity progress. After every 1000

partial cycle an electrochemical

impedance spectroscopy for the investigation of impedance development, in or-

der to interpret ohmic, charge transfer and diffusion behavior is carried out. The

third part of measurement procedure is the cyclic voltammetry (CV). Based on

CV the mechanism of different degradation behavior is discussed. The potentio

dynamic measurement allows conclusions about electrochemical behavior con-

cerning the kinetics and physical processes on the electrodes [33].

2.1. Cycling Tests

The cycle tests are executed with a battery test system Basytec XCTS. In order to

control the ambient temperature a climate chamber CTS T-40/350 is used. The

C. Betzin, H. Wolfschmidt

DOI:

10.4236/msa.2018.91011 158 Materials Sciences and Applications

SOC is adjusted via Ah counter with an accuracy of ±0.2%. In this study two av-

erage states of charge are considered. The first state of charge is at 30% SOC with

an initial open circuit voltage of 3.53 V and the second cycle test belongs to the

state of charge 70% (OCV of 3.75 V). For each cycle test two cells are considered

and the current rate is C/3.

After every 1000

partial cycle with a DOD of 10% executed via an Ah-coun-

ter a characterization cycle is carried out. That characterization cycle is done for

capacity determination and readjusting the SOC, which was not necessary dur-

ing the whole testing time. In order to determine the capacity loss two discharge

capacities are specified. After the common CC-CV charge cycle with CV poten-

tial of 4.0 V and the abort criterion of C/20, a discharge with CC of C/3 like

charging is done. Subsequently a constant voltage phase at 2.7 V with the same

abort criterion like charging is carried out.

Table 1 shows the comparison of the

cycling conditions.

2.2. Electrochemical Impedance Spectroscopy (EIS)

After each 1000

cycle a galvanostatic electrochemical impedance spectroscopy

(EIS) is carried out to interpret the capacity loss behavior. The impedance spec-

troscopy is done via multi-channel galvanostat/potentiostat EC-Lab Biologic

VMP3 and a current Booster VMP3 B-10. The ambient temperature is con-

trolled by the climate chamber Binder KB53. The impedance spectroscopy is

used to determine internal resistance and interfacial capacitance of cell. The ba-

sic of this measurement is by reference an AC source to determine the imped-

ance depending on frequency by varying it. The measurement allows interpret-

ing equivalent resistance and interfacial capacitance values by means of correla-

tion to electrode interfacial phenomena [34]. In this study the carried out band-

with of frequency is 6 kHz up to 50 mHz with 36 points per decade in logarithm

spacing. Per each frequency an average of two measurement points is carried

out. The galvanostatic impedance mode is carried out with an amplitude of 1 A.

Table 1. Cycling initial condition (voltage limit) at 25˚C ambient temperature with 10%

DOD at SOC 30% and 70%.

30% SOC 70% SOC

OCV 3.53 V 3.75 V

Charge (C/3) to 3.62 V 3.85 V

Discharge (C/3) to 3.44 V 3.65 V

1000 partial cycles with 10% DOD

Charge CC (C/3) to 4.0 V

Charge CV 4.0 V (C/20)

Discharge CC (C/3) to 2.7 V

Discharge CV 2.7 V (C/20)

Capacity determination

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