Question 78·Hard·Inferences
To curb capacity fade in high-energy-density lithium-ion batteries, engineers investigated two electrolyte additives—fluoroethylene carbonate (FEC) and dilithium dithionate (DTD)—designed to stabilize the cathode-electrolyte interface (CEI). After 300 charge-discharge cycles at , cells containing 2 wt % DTD retained 88 % of their initial capacity, whereas cells containing 2 wt % FEC retained only 72 %. Subsequent spectroscopy revealed that CEI layers produced by DTD were thicker yet noticeably more uniform than those produced by FEC. Taken together, these findings suggest that ____
Which choice most logically completes the text?
For “Which choice most logically completes the text?” questions, first underline the key evidence just before the blank and note any summary phrases like “Taken together” or “As a result,” which signal that the blank should logically follow from what was just stated. List in your own words the main findings (here: comparative performance numbers and a structural difference) and then look for an option that (1) uses only that information, (2) does not contradict any data, and (3) does not introduce new, unsupported ideas. Eliminate choices that speculate about untested conditions, add mechanisms or factors not mentioned in the passage, or reverse the cause-and-effect implied by the results; then select the remaining choice that most directly connects the given evidence.
Hints
Locate the key phrase
Focus on the sentence with the blank, especially the words “Taken together, these findings suggest that…”. Ask yourself: which two findings are being combined?
Compare the two additives
What numerical result is given for batteries with DTD versus FEC after 300 cycles? And what structural difference between their CEI layers does the spectroscopy reveal?
Think about what is and is not mentioned
Do any of the choices talk about things like higher concentrations, temperature changes, or general rules about thickness that were not described in the passage? Be cautious about answers that introduce new information.
Link structure to performance
Ask: Which answer choice directly connects the observed difference in CEI structure to the observed difference in capacity retention, without contradicting any data?
Step-by-step Explanation
Understand what the question is asking
The sentence ends with “Taken together, these findings suggest that ____.” You are being asked to infer a conclusion that logically connects all the given data: the capacity numbers and the structural description of the CEI layers.
List the key findings from the passage
There are two main findings:
- Performance result: After 300 cycles at , cells with 2 wt % DTD retained 88% of their initial capacity, while cells with 2 wt % FEC retained only 72%. So DTD performs better than FEC.
- Structural result: Spectroscopy shows that CEI layers formed by DTD are thicker yet noticeably more uniform than those formed by FEC. So the key difference highlighted is uniformity (DTD is also thicker, but still performs better).
Decide what kind of conclusion is reasonable
The phrase “Taken together” means we must combine the performance and structural results to propose a plausible explanation, not something totally new.
- We know DTD cells perform better and have a more uniform CEI.
- We are not told anything about changing concentrations, internal temperature effects, or general rules about CEI thickness.
So the best conclusion must:
- Connect CEI properties (especially the emphasized uniformity) to
- Reduced degradation / better capacity retention in the DTD cells.
Match that reasoning to the answer choices
Now compare each option to what the passage actually supports:
- (A) suggests changing FEC concentration, but the passage reports results only for 2 wt % and provides no evidence about higher concentrations.
- (C) brings in a general claim about thicker CEIs impeding transport and then predicts DTD should lose more capacity; that expectation conflicts with the stated results (DTD retains 88% vs. 72% for FEC).
- (D) proposes improved retention comes from lowering internal temperature, but the passage never discusses the additives changing cell temperature.
- (B) directly connects the more uniform CEI formed by DTD to reduced cathode degradation, explaining why DTD cells keep more capacity than FEC cells.
Therefore, the best completion is:
“the uniform CEI formed by DTD is likely what mitigates cathode degradation, allowing batteries with DTD to preserve more of their original capacity than batteries with FEC.”