Articulation is essentially saying words clearly. If you want to be understood when you sing, you articulate, however it can be overdone. One way to over-articulate is to tighten your lips and stretch your jaw wide to emphasize consonants while you sing. This can tighten your tongue and the muscles in the back of your throat. In turn, your larynx, which houses your vocal folds, is stiffened and made inflexible. This hypertension (hyper = too much) prevents your inner vocal apparatus from moving easily.
When the larynx is prevented from moving freely it's harder to sing or reach higher notes. This is a self-imposed difficulty. Yes, you have to move your lips and tongue to form your consonants. However, you do not need to exaggerate the positions of lips and tongue in order to be understood. The vowels, not the consonants, should be the heart of your sound. The best way to sing includes the relaxation of the lips and jaw. You can do this and still articulate.
To demonstrate this let's use the word "How." Choose a pitch slightly above your usual speaking voice and sing the word "How" as follows: Push on the "H" and hold the pitch as you tighten into the "W." If you sounded like a wounded dog, you did it correctly. You probably also noticed your voice tighten and the sound quality was not as pleasing as you might like. Try it again. Really exaggerate the "H" and the "W." OK, I'm sure you get the point that this is not a good way to sing.
Now try it again differently. Think of the actual sound of the vowel of this word and create the pitch you chose with that sound. The vowel sound in “how” is an "A" sound pronounced as in the word "Apple." Try it with that sound in mind. You should find that the "H" and the "W" naturally reduce without your having to really think about it. Just determine the vowel sound (not its letter name, the sound of it as you would say it) and sing that on the pitch.
This would be your mental focus, while still singing it in the context of the word itself. You should find yourself singing it more freely and sounding much better. I have a variety of exercises that would help you with this within my self-study course: The Contemporary Vocalist Volume I. In our next lesson we will continue our exploration of expanding vocal range.
All the best,